I like the idea of Rosalie changing after Nessie is born too. I don’t believe she was around long (I’d have to look it up) before she found Emmett, so I think she changed fundamentally by being given the chance to “mother” Nessie and that leaves her feeling complete. She and Bella formed a bond that will last because first, Bella needed her specifically and then because Bella, although inadvertently, gives her the chance to explore that part of her nature.
I’m not sure that Rosalie and Edward have the same view on what they are. Yes, they both believe they are monsters and they both hate it, but I think even that’s for different reasons. Edward believes that he’s one of the “eternal damned” but I don’t see that Rosalie thinks that deeply on the subject. I do think that the vanity plays into it more for her in that she’s does pretend to be human more but I think that if she could have children, she honestly wouldn’t have been unhappy with being eternally beautiful. Her line in the movie, “Emmett gray haired by my side...” doesn't seem like something that she truly wants. It doesn’t fit with her nature to want to grow old really...
For Rosalie, I think her biggest issue, wanting what she can never have. She was young and beautiful and had her “life” planned out before her to get everything she wanted. Then this tragic event happened to her. It would’ve made sense to me if she had been angry at Carlisle for turning her, but I don’t think she had an issue with leaving her real life behind. I think it was more that the life she should have had was stolen from her by Royce and his friends. Had she not been turned her life would have been over for her. She couldn't have lived with the results of her assault, even if she had somehow survived physically. I think that is the part of what she is that she has never reconciled with...and the reason that she thinks she would have rather died.
As far as her reaction goes, it’s stated more than once that something of your nature is sometimes carried forward and amplified when you are changed. For Rosalie, maybe she wasn’t quite so “unlikeable” as a human...though it seems as though Edward really didn’t like her to start with. He thought her selfish and spoiled so I’m not sure her reaction held any bearing over what he thought, with the exception of Bella being changed. Her reaction to being changed was the strongest so to Edward's way of thinking, that's the one that would show the most contrast to Alice, who doesn't seem to mind being a vampire and is very close to Bella. I think he was grasping at anything that would make her change her mind or at least make her wait. Maybe he was trying to get Bella to understand that she wouldn't necessarily be growing old so much as having the chance to grow up. In my mind, Bella's reaction to growing old seems very shallow considering how comfortable she is with herself in every other aspect.
I’m not sure even he realized that because he pushed Rosalie’s reaction so much to Bella that she knew exactly who to call when the pregnancy happened. If ya’ll remember, Rosalie thought he’d be mad at her for telling Bella her story. He only thought that it would give her a perspective that he couldn’t explain to her. As usual though, he didn’t anticipate the way Bella changed her view of Rosalie after hearing it. And instead of her story allowing Bella to realize what she’d be giving up, as Edward hoped, it only made her more stubborn to get what she wanted before. Can you imagine what Edward thought towards Rosalie during the pregnancy? Especially at the beginning, before Jacob got there, which I think helped him alleviate some of his frustrations about her. Jacob could get away with threatening Rosalie and making snide comments. Even had Edward thought the same, he’s too much of a gentleman to voice those opinions.
After the wedding and the birth, Edward seems content with what he is. I don’t think he’s as conflicted as he was and I believe that’s completely due to having someone to share forever with. Rosalie had that previously but what she didn’t have was a family. During the pregnancy and after, I think she and Edward both were able to reconnect with their humanity which is what they were missing from their lives. They were just missing it for different reasons.
What would be the reaction to my theory of having Leah imprint on Nahuel? Would that be a conceivable (no pun intended) scenario?