Well in Volterra he said: 'Carlisle's right!' Referring to the 'vampires have a soul' theorie from Carlisle. The moment he said that and i realised what he ment everything changed! It means that he still has hope and doesn't really completely believe that he has no soul at all! He was so stobbern about it because he didn't wanted to doom Bella and give her the same fate as he has, only because he is selfish enough to keep her. So if he can believe vampires have a soul there isn't a reason why he don't want to change bella.
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?
This is a hard one. Can't really see what Edward means with it.. But in his own time it was normal to get married so jong that counts to. And I think he wasn't joking about that Bella has to 'prove' that she wants to stay with him forever. It's one thing to stay with somebody a lifetime but how does Edward know if Bella will stay forever! He wants to be as sure as he can get and i don't blame him for it.
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 think it's for a big share the shock, that she reacts how she does. He completely took her off guard about this, Bella never thought about this and that isn't a surprise. In this time it's not completely normal to marry at 18. She keeps hanging in a denial stage, thats why she is so horryfied about it!
4. After the epiphanies of NM, how could Edward still imagine being immortal was more important to Bella than her love for him?
People do crazy things.... Also they think weird sometimes... Edward is really complex he thinks about everything! So it's not a surprise he thought this one out to.

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?
Yes it's selfish and yes he is weak that he can't resist to doing it. But o well what would you do? I can be really selfish everybody can be, sometimes you need to be. If someone says that there not or never selfish they are probably the most selfish people alive. If people say they are selfish they really aren't but they have to do something that is selfish but they don't like to do it. And well Bella wants him to do it so that makes it right.