1. Does Bella really understand all that she will give up in becoming immortal? Do you think her understanding is limited by her age, life experience, or something else?
No, I think she understands it all. she didn't in the beginning of the series, but now, after Eclipse, I think she does. Age wouldn't limit he - remember how renee called her a middle-aged child?
2. What do you think Emmett and Jasper were going to do for Edward's bachelor party? Was Jasper telling the truth?
I can''t imagine them doing anything, probably just hunting. Maybe they drove really really fast somewhere.
3. Tanya made a play for Edward long ago. We don't know much about Tanya, but we know quite a bit about Edward. Why do you think he turned her down?
Edward just didn't see her that way. End of story.
4. About Immortal children. At what age or developmental stage would the Volturi allow them to be created? A two-year-old usually understands the word no, but evidently that wasn't enough. How would they determine if the child could "keep the secret"? What would the determining test(s) be?
Probably no younger than ten, and it would depend on the individual kid, and how restrained they were with other stuff as a human.