1. Why is Edward now willing to change Bella into a vampire (provided she marries him) after being dead set against it for so long?
Because it is really what he wants too, to be with Bella forever
2. Why is it still so important to him that she marry him before he changes her? Is there more to this than a strategy to delay her?
He wants Bella to belong to him and he wants to belong to her in a way that everyone can see.
3. Why is Bella so panicked at the thought of marrying Edward? Is there more to her reaction than she is saying? Does she actually understand her own reasons completely?
Just for the reason that she believes her mother will flip out about marriage right out of high school.
4. After the epiphanies of NM, how could Edward still imagine being immortal was more important to Bella than her love for him?
i am not sure, it baffled me too.
5. Edward says that changing Bella is the most selfish thing he will ever do. Why is he nevertheless prepared to do it? Is this morally weak of him?
He sees that being immortal is what Bella truly wants, so now he knows that it is something they both want, to be together forever, I don't think that it is weakness at all.