See if she would want to go at all. Maybe it would take her mind off of losing the baby. If not, hopefully a friend could come through. As sad as it is, it sounds like you may not get another chance any time soon to celebrate your brother's wedding
Maybe if I'm in a shared space, but the rule was 'in ANY room' meaning even in bedrooms, which are private. Nope, she can't enforcement that one. Or the PDA one. That could easily be as simple as being cuddled up on the couch with your partner. Nothing wrong with that. Of she has a problem, she can easily not look.
Nope. No, no, no. All that would do is convince Barbara that he buys into her and her mother's delusion. That would cause a HUGE problem when OP fades from her life again, because he has no connection with her. He has no reason to stay in her life, and he probably won't. This would send Barbara on another spiral of feeling 'abandoned' by her father, "AGAIN", when it never happened the first time.
This would be the WORST idea, walking her down the aisle. If he wants to be a father figure, he can start at another time, not when there are expectations put on him.