I'm suffering from the fact that I forget the details of books really quickly now, so please excuse me if my answers are very inaccurate!
5. Several characters, especially Ram and Salim, are big movie fans. Is there a reason for this? Do films help them escape their frequently dreary lives, is it simply a significant part of their culture, or is there another reason?
When reading the book, I very much thought of them seeing films as pure entertainment to take their minds off the awfulness of their daily lives. But thinking about it a bit more, of course film is a huge part of Indian culture, too.
6. What are Ram's ambitions in life? Why does he tell Prem Kumar he doesn't know how he's going to spend the billion rupees?
I don't remember thinking much about Ram's ambitions in life, although to me his aim was to find someone to love that would love him back and provide the sense of belonging that was missing due to his being abandoned. The only thing I can remember about how he was going to spend the money was to buy Nita's freedom from prostitution so he could marry her, and maybe that was a bit too personal to want to talk about.
7. Why does Ram turn in Colonel Taylor? Is this retribution for the colonel's spying, his derogatory comments about Indians, or for the way he treats his family? Or does Ram simply want to collect his wages before returning to Mumbai?
I certainly wouldn't have been surprised if Ram had turned him in for any of the unpleasant ways in which he behaved that are mentioned above. But when I was reading I just thought the time had come that he needed to collect his wages.