A: It depends on how you both act. If you flirt with her/show some type of sign that you're interested, the crush is going to continue. It's all about fanning the flame.
A: I would suggest some kind of joke. Or something light to break the silence. I think it really depends on the direction of the conversation. You can always ask questions. Guys love to talk about themselves!
A: I would let him know the circumstances and explain to him that this is purely for fun. I think guys more so than girls are into a "friends with benefits" kind of thing, so he probably won't object. Just don't lead him on.