1. I certainly think that it would be possible. I would have loved if Jacob and Tanya got together, but Leah might be really sweet with a vampire. I've read a few really cute Leah/Nahuel, though he's not a full vampire.
2. I don't know. I'd like to think that they wouldn't turn human, though I don't know about Rose. Maybe she would be happy enough to have Nessie and not turn back. If she turned back, then Emmett would too. I can't imagine Bella turning back, so Edward wouldn't, nor could I imagine anyone else turning back.
3. I'll have to get back to you on that one. I'm not sure. I think it would be absolutely epic, but specifically, I don't know. Though I would love the song, "As Long As You're Mine" from the musical Wicked as a duet between Edward and Bella.
4. I don't think Alice would be able to resist telling Cynthia if she saw her, so I probably would guess that the family would stop her from doing anything else besides just seeing her from across the way. I think that would be an awesome fanfiction- where she meets Cynthia!