Unfortunately I had to edit your original question because enotes only permits you to ask one question at a time. Multiple questions are strictly forbidden. Please make sure you only ask one question in future.
As regards to the believability of characters, to be honest I see no indication that the characters in this excellent novel could be anything but believable. They are presented as having the same kinds of questions, fears, worries, hopes and joys as the rest of us do. In particular, I think the presentation of Jonas and the way that he has to come to terms with the privileged position that he occupies in the novel and how this separates this from his friends and ends his life of carefree ease and innocence is presented very well. Consider the following quote:
Jonas trudged to the bench beside the Storehouse and sat down, overwhelmed with feelings of loss. His childhood, his friendships, his carefree sense of security--all of these things seemed to be slipping away. With his new, heightened feelings, he was overwhelmed by sadness at the way others had laughted and shouted, playing at war... He felt such lover for Asher and for Fiona. But they could not feel it back, without the memories.
We are given an insight into the intense loneliness that Jonas now feels and how he feels cut off from his former friends and the rest of the community of which he had felt such a part. Such details as this help present highly believable characters that we can all relate to and sympathise with.