The impact of that initial period after my wife came out, was the most dramatic event in my life. In 2005 the possibility of homosexuality in a straight marriage was a lot less known than it's nowadays. Realizing the consequences took some time to me. Possibly this was an advantage, to me the shock was spread out over a couple weeks as I started to take it in. It's a severe emotional trauma, I totally affirm that. But how people react on this differs. 
I do recall incidental physical reactions, like shaking and uncontrolled emotions of grief getting the better of me. Sleepless nights and mood swings. Sometimes thinking we could overcome it easily, feeling confident, a new look and buying new clothes. Only to land with two feet on the ground some time later, when I realized it wasn't reality. But overall I tried to suppress my emotions by trying to "think my way out of it", somehow getting control by using my rational power. That's my personality trait. And on itself this was useful in some respects, but I buried my emotions as much as possible "for the good of the cause". And that didn't do me good either. All that stress build up inside which eventually took a toll on my health. 
So letting emotions out is not a bad thing, also talking about what it's all doing to you is a good thing to do. Try seeing a therapist or confident asap, talk and let it out!

At the same time, it's helpful to lessen the threat you fear. As long as that fear exist, you stay in red alert mode, feeding your emotions. So try to get yourself to more save ground. As I wrote in an earlier post, please stop reading dread stories with negative outcome. It's not helping you and it blocks a realistic vision on your situation and your way forward together. You're both in it. It's beautiful when you and your wife bond up to face the situation and go for the solution you both want to achieve, it brings you closer together in the process. Try to think positive, there is hope. 
It's not easy, you and your wife will have to learn and evolve, but I assure you it is possible. There is no natural law that forbids it, it all depends on you and your wife. The persons you are, the love you share and decisions you make, it's free will that chooses. Like finding a path through a mountain range. Looking ahead for the best route, avoiding steep rocks and watch your step on the slippery parts of the path. 
My wife and I have found the way, we're on top of the mountain so to speak. From this viewpoint we have some understanding of the rugged terrain behind us. I don't know if you can see us, but we can wave flags to signal some of the direction to go to...

Maybe it's constructive to address your current obstacles one by one. Let's discuss each aspect carefully, before moving on to the next. 
I'll tell about the way we experienced it, handled it, coped with it. Feel free to ask anything, and dig in deeper if you want. You're totally free what you do with it, I will not try to convince you of a point of view,  just telling about our experience (good decisions as well as mistakes). If you think this is a good idea, kick off... and we'll see where it leads us.

We were engaged in the early eighties, our relation seemed normal enough, the thought of possible homosexual feelings never crossed my mind. But even when it would have, at the time my wife wasn't aware of those feelings.
Marriage is in sickness and in health, for better and for worse. That's the agreement we entered in and I said "Yes". Not knowing beforehand what will happened during life, and that's precisely the intention behind this promise. 
Ten years into marriage, my wife went through periods of deep depression and alcohol abuse, that sure didn't make me happy also (four young children to care for). Should I have asked before marriage if she was expecting to be depressed and do all that? Of course she didn't know nor that she later would discover to be lesbian. These things weren't by choice, it wasn't predictable, it just happened in our life.
So even when we would have had discussions about these matters before marriage, it wouldn't have made any difference.

About five years before my wife came out (our sexual relation wasn't going like it should) we asked ourselves whether these problems could be caused by her having a non straight sexual orientation. But there were no positive signs that would be the case, so we dismissed it. Even then she was honestly not aware of her feelings. That's just the way it is, nothing one can do about it.

When my wife told me 15 years ago, I knew this wasn't by choice, and accepted that as a fact we had to deal with. Certainly not something to blame her for. 
And of course I questioned her about it, did she knew when we married? But she didn't, she wasn't aware of it. 
So even when it would have been a topic to ask her before marriage, it wouldn't have made any difference to my reaction.
To me it was: okay, were confronted with serious and complex problems, but she's still the one I said yes to and whom I love. 

It certainly would have made a difference if she had known consciously about her feelings before marriage. Not the lesbian feelings itself would have enraged me, but the fact that she would have lied to me and deceived me for 20 years about a very important matter and hid the reason for the sexual problems we went through. I'm quite sure that would have created a totally different situation, and a very different reaction of me. 

But this wasn't the case. She didn't made a deliberate choice to have these lesbian feelings, I can't blame her for it. We're both in it, we still choose each other, and we'll deal with it together as good as we can. The fact my wife wanted to continue with me in our marriage and I did too, because we loved each other, was the most important. Next thing of lesser importance, was what it all meant to our relation.
I didn't know what it all would would bring or what future was ahead. Only thing I knew at the time was this was the right and loving choice to make.

Though my wife probably would have known about her desires if she let herself, I don't think she intentionally deceived me. 

You'll never know "what and if" for certain, it's guessing. This isn't important. Stick to certainties and reality, that's more than enough to work out. 
She didn't know, period. She didn't deceive you, period. It's really important to keep your focus on reality. 

Did jealousy ever become an issue? I already find myself being anxious and jealous over my wife's attention to women (real or imagined), whereas I never really felt that way about other men. 

Hmm, jealousy, a strange feeling that is. It's not a feeling that exists on it's own, it has a reason. It's connected to something else, more basic.

Initially I felt jealous, but actually it was really all about rejection. If my wife was lesbian, then what about me? If she actually wants a woman to be with, there is no way I can compete with that. 
It's totally different competing with a man. Because then you have your comparable aspects and properties... well, bring it on! But when your gender as such is rejected, nothing really matters anymore. 
It's just no comparison, a man cannot compete with a woman, and a woman cannot compete with a man.
What about jealousy in such a situation? It's not just that, it goes beyond normal jealousy.
It addresses the question what your spouse wants at a basic level. Rejection is not about some negative or positive properties you have, but being rejected for what you most basically are.
Jealousy is the label we put on it, but the feeling of rejection of whom you basically are is what it's really about.

How did you overcome that?

I never did as such. It's not something you can overcome by yourself. Because it simply depends on how your wife experiences you and what she wants. 
If she wants a woman and nothing else, then that's how it is. But if she wants you instead, then that's how it is also!
Not that I was waiting in the corner for her verdict, I have the same choice. When I choose my wife, that's my will. Not just because she is female, but because I want her.

Regarding this aspect, I was thinking and considering a lot, mostly in myself. Comparing our situation to internet stories, trying to understand, the works. 
Meanwhile I felt rejected but not because anything my wife said or did. I myself had a very hard time accepting that my wife wanted me. Against reality because she had decided that long before. She didn't want to pursue her lesbian feelings and wanted to continue our relation. For her it was reasonably clear, but not to me. 
It's complicated (or better, I made it complicated):  "I know what I think, but for me there's that uncertainty factor in what my wife thinks"... 
Probably for me the lack of trust was the reason. The whole situation in the first year after ground zero had shook my trust. But also my personality had much to do with it, I'm not the kind of person that likes to take things for granted. I have to "understand" first. So this kept me tossing and turning for a very long time. 
Actually I had to come loose from this feeling rejected, for it blocked our way forward.

Back to your question: Jealousy went away when I accepted that my wife wanted me. This went hand in hand with rebuilding trust in that.
So that's the central thing: rebuilding trust. 

Once trust was back, the whole lesbian feeling thing didn't bother me anymore. 

She has gone from having my absolute trust to being no longer trustworthy (she denied her feelings within herself, denied them to me (even during over a dozen serious discussions about the topic/possibility over the last year), spent 2 years indulging in secret and planned sexual activity with women (them being web women makes little real difference), and she did all this during a period where she knew I felt sexually rejected).

After my wife came out she still held on to the feelings for the woman she fell in love with. Even though she knew it would hurt my feelings, but on the other side she wanted to stay married with me, and chose to go for that. At the time I didn't understand (nor accept) her behavior, and rightfully so, of course it wasn't okay how she handled things. Rationally she knew this also, but emotionally she felt that need. 
To me this was incomprehensible, and so different than the wife I thought to know so well. It was so very different than the image I had of her.
(in our case it wasn't porn, but pictures of the woman she kept secretly, playing popsongs that reminded of her, driving by her house hoping to get a glimpse of her, those kind of things. Even though she was aware of my objections to that, she did it secretly.)
My thoughts at the time: if she cannot control her emotions and behavior, while it is so obvious that hurts me, how will I ever find insurance that this won't spiral out of control in the (near?) future.
The unconditional trust I had in her was damaged.    

At the time my wife was confused also, discovering this new side of herself, was a lot for her too. Her thoughts were centered about what she went through herself, not so much what it meant to me. That's why she allowed herself behavior that was very unlike her. (years later, looking back on that first period, she comments about herself that she wasn't acting sensible and let herself go way too much).
Our skills to talk openly and effective had still to develop. So we both weren't perfect and provided the ingredients for a troublesome time. And I think this is unavoidable but also surmountable.

She is afraid that it won't matter what she says, I will not believe her and leave. I have to admit that is an actual possibility, though I would honestly feel something like death would be preferable. However, if I have to choose between two terrible things - divorce now or being lied to until she becomes more self-aware and leaves me in 10 years, I will choose divorce now. My hope is that I won't have these as my only options after we talk some more.

If you discard everything she says because you fear it are all lies, talking wouldn't help much. Then you've cornered your wife as well as yourself. It would be a bad thing if the negative stories you've heard, should block any solution beforehand. Fear is a bad advisor. I sure hope you can (force yourself to) put that negative thinking aside and focus on reality.

Although your wife watching lesbian porn feels like cheating to you, I think it is not quite the same as physical adultery. Because when she actually would have had sex with a woman, would that really be the same to you? I guess not, for that would have had a lot (!) more impact on the whole situation. Not only on you to overcome such a situation, but I think it would have changed your wife also. Deciding to and taking such a dramatic step while being married means quite a lot. Also the experience would never leave her memory. So it would be much more a burden if she should have done that.

She is conducting a serious self-examination about what she desires sexually, why she hid it from me, why she hid it from herself, and why/what allowed her to act so deceptively regarding her use of porn and for so long (when she knew it would hurt me terribly)

This self examination is a sensible thing to do. 
Her motivation to watch porn is very important. Was it in her mind a way (or first step) to cheat on you or intentionally deny you having sex by finding another outlet? (say, bad and negative motivations)
Or was it (just) about discovering/acknowledging aspects of her own sexuality for example? Maybe confused about it herself, but finding it exiting, a bit ashamed of it. Not something to share because she feared it would be problematic to you? 
And though it wasn't ideal she kept this a secret for two years, this isn't nearly as problematic as those really bad and negative motivations. It just shows that the communication between you two (real openness) wasn't there yet. It may feel like a sort of cheating to you, and that´s probably because it triggers your feeling of rejection, but her motivation is the most important thing to consider.

Confused about new sexual feelings (keeping a part to herself). Insufficient communication, which fuels doubt about what's really going inside. Trust is damaged. Feeling sexually rejected. Your wife faces a situation where feelings she didn't choose and only partially understands herself, are hurting you. She's stuck, hiding it didn't work out well, telling about it didn't work out either. She feels rejected for who she is. 

A list of points, you both have to work on, can be easily derived from this description. And that's the way ahead!
I can imagine it looks overwhelming at this moment, but you can also see it another way: when you've worked it through, it's obvious your relation will have gained a lot. By then it's on another level.
And you have started this already: your wife's self-examination and you both talking about it. 

As she admitted, she knows that my previously unassailable image of her honor is totally broken and will have to be rebuilt from scratch.

Your wife isn't perfect, she has unexpected sexual feelings, she made mistakes handling those, she has to learn about herself. And you can learn to love the real woman she is, not only the image you had. She can feel accepted and loved just the way she is. What you rebuild doing that is more than what was before.

When we look back, it was very worthwhile to us and in so many aspects. We wouldn't want to go back to the situation before we entered `the MOM`. Sure I wish it had been easier to find our way through it, but often it needs real difficulties to make real changes possible.

I appreciate your feedback a lot.

The reason she was going alone was because I thought she would be more comfortable that way, not having to figure this out in front of me, having to divulge half-baked thoughts and feelings with my inquisitive mind and critical ears analyzing everything she says. I and she are both a little worried about what would come of me being witness to her statements or the therapists suggestions, until she is more certain how she feels. Does that make sense? I have a strong tendency to hear everything she says and see if/how it lines up with everything else she has said or done (that's how I discovered her bisexuality before she did).

Your wife and you think it is better for me to attend her sex therapy sessions, while she works out what she desires sexually, why she hid it from me, why she acted on it, etc? I think the therapist also suggested it would be better if I wasn't there, at least initially.

We stopped going to couples therapy because (1-two therapists is too expensive; 2-I wasn't real pleased with the therapist we have [so little feedback, plan setting, strategies, etc.])