Well, you could look this up yourself, but… 1. Free 2-day shipping only applies to stuff sold by Amazon, meaning that most of the “used” books and such that are sold by other sellers (the $.01 + shipping stuff) aren’t eligible. For titles that are pretty common, though, amazon sells then cheap enough that it balances out with free shipping. In order to get an idea for how it pans out, modify your searches to only show “prime eligible” items.
I’d say, it’s usually worth it. 2. yeah. 3. aside from, like, rules and such, i’d imagine it should be pretty easy to share your account with a fellow student, provided you’re not worried about them having access to your $$$ if you’ve got a card or bank account directly linked.