Sounds like you've put the whole wedding hullabaloo into better perspective then. Good on you!
Weddings ARE stressy and DO eventually blow over. Don't sweat it.
If the bottom line was about finding some reason NOT to have you there because of not being out to all relatives -- we get that. who doesn't?
But I note you write this:
Disapproving of my marriage? I don't think so. But I know that M wishes I'd assert myself more. Come to think of it, R says the same thing.
I said it a few times in my posts --- get more firm and assertive. They cannot mind reader you if you are to passive in your approach. That gets frustrating.
We teach others how to treat us.
Google "conflict style assertive passive aggressive" and see how each is different.