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?
At the end of New Moon, after the family vote, Edward knew he had no choice but to change her. It was only a matter of how long he can stall her. He wanted her to experience as much human memories as she can, college, etc... so I think after much debate between them, he finally caved and agreed to do it provided they get married first, again so Bella does not miss another human event.
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?
Edward still stands by many of his 1900 principles, Marriage is very important to him, as a 17 year old, if he was not so into enlisting and going to war, he would have found a girl and made it his obsession to win her hand. He wants Bella bound to him, not by need or danger of the Volturi, but by her own choice, which would show by her decision to make her his in every way, legally and eternally.
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?
I never understood why she was dead set against it, when she was willing to give up humanity for him. I don't think she understood herself.
4. After the epiphanies of NM, how could Edward still imagine being immortal was more important to Bella than her love for him?
The fact that he can't hear her mind. Edward is still full of insecurities and self loathing that he cannot imagine why in the world Bella could love him as much as he loves her.
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?
No. He must see that changing Bella was inevitable. Everything was pointing in that direction. Yet he feels selfish, as he knows this would make her his forever, and he still feels unworthy of that.