If you have decided that you have to go to this wedding, then maybe this can help.
I have found it helpful through this whole ordeal to list things in two categories: those things I have control over, like tasks to do, and those things I have no control over. I tackle them separately.
So, can you make a list of those things that are in your control, tasks you just have to figure out how to do? Like, reserve the hotel room, contact someone to ask if you can hitch a ride with them from the hotel to the reception, and someone who will serve as your "buddy" while you're there, someone you can check in with, even if you don't sit with them? Those are tasks, something you can take control of. Taking control will help you feel more in control.
As for the things you have no control over--like your friend finking out and your husband's wigging out--those things you have to confront another way. Yes, it's intolerable to live in someone else's closet and to live a lie in public; it's especially difficult to go to a wedding when your own marriage is falling apart.
Maybe you could reframe things a bit in a way that will help you get through the event. There are likely to be other people at that wedding for whom a wedding will also be difficult: the recently widowed, for example, or those whose spouses are cheating, who have just divorced for reasons that aren't their fault, or are in a marriage suffering domestic abuse. Your situation is different than theirs, but I guarantee you won't be alone in seeing the wedding as a challenge.
As for the lies you have to tell to keep your husband's activities secret, the fact is that as long as you are determined to continue to live with your husband you will be faced with this problem, so you will have to develop a script for yourself and a repertoire of phrases to cover over the truth. You don't have to over-share, either: in this case, a simple "we had problems getting a sitter" will suffice.