Hi,
I can't answer all your questions.... but I can tell you that if you want to train in the US I have been told by several from the UK its much cheaper. At your age I would go to a flight academy where you can build time as a CFI. It would be quicker that way. Also I am a CFI and I have been working with someday from the UK who has their JAA PPL. He needs to transition to a US license. He has a visa too. He had to go through the TSA on a new rule they have for security before I could start flight training with him. He needed to get finger printed and it was quite a process. I will try to get you a link to the web site were you register later today.
Chris