Actually, I now think my advise to reply to him is wrong. I think it sounds like he's not actually upset about seeing you at the wedding, but more is liking what little control it gives him over you/the situation again. If he's not got over it (which is what it sounds like) for 2 years he's had to just get on with life and not be able to force you to discuss it or have any excuse to let you know how he feels. Now he's making as much fuss as he can because this is the only chance he's got. Trying to force you to discuss 'what went wrong' before he'll "allow" you to be BM for someone else is not about not wanting you there, but trying to use someone else's wedding to run his own agenda.
What you should do is forward his e-mail to the bride (and groom if possible) and say:
"hi [bride]
Please see the message below from [exH]. I'm very tempted to give him a piece of my mind, he seems to be trying to use your wedding to force me to hash over "what went wrong in our marriage" and if he doesn't like the answer, he feels he has a right to decide who you have as your bridesmaid!!
I'm going to tell him to do one. Not having [DP] at the wedding in order to keep [exH] happy was disappointing, but both [DP] and I know you were in a difficult position, but now to say he's going to 'ban' me being your bridesmaid when you've already bought dresses etc is not on, particularly as it's not like it's his choice who you have at your wedding!
However, I thought before I told him this I'd check what you would like me to say, or if you or [Groom] would prefer that I said nothing and you have a word with him. I am concerned if we ignore this little 'guestzilla' outburst, he'll make a scene on the day, and in that case, it might be better if I do step down as bridesmaid if it's not possible for him to act in a civilised manner for one day - I'd hate for anything to ruin your day.
Let me know your thoughts,
OP."
You might find they've decided they have had enough and while your DP not being there didn't upset their plans too much, or you leaving before the evening do (when you'd done all your bridesmaid duties) wouldn't impact that much on their day/plans. But this does mean wasting the money they've spent on dresses, possibly dealing with embarrassing conversations about why OP is now suddenly not a bridesmaid (when a lot of family and friends will already know who the bridesmaids are) - that might be enough for them to stop pandering. If it's not they aren't really your friends and are rather weak people...