
1. Easiest: My gut reaction would be Carlisle. He's well-read, been around for a while, could possibly have a common ground with Jasper in history (as in, their interest in it). Esme is also a possibility, depending on whether or not you think he'd appreciate the motherly nature she exudes.
Hardest, I will say Emmett. Purely because Emmett's normally jolly demeanor would go against Jasper's very being- why is this guy having an easier time than he is (at abstaining) and being all happy all the time?

2. I imagine he would have attempted to start the moment they joined the Cullens. As to whether he's been able to successfully stay on the wagon since then, that's debatable. Though with both Alice and Edward around, I'm fairly certain the chances of a slip-up are rather slim.
3. Probably amazement, at first, seeing her integrate so well with them. Perhaps a bit of jealousy, partly because she's able to fit in and he doesn't quite get there, and partly for seeing her establish other relationships (with Edward most likely)- until he understands it's not romantic, at least.