All of the schools you've been accepted to have good reputations, but you need to be very realistic about your options.
As an international student, if I want to practice law in US after graduation, I have to go to those biglaws sponsoring H1B visa. So employment rate at biglaws is one of the most important factors for me to make decision.
Hiring at the big U.S. firms is extremely competitive. You will most likely need to be ranked near the top of your class, which is no small accomplishment in law school. Your classmates will be just like you: disciplined, motivated, and smart. Everyone will be striving to reach those coveted few spots, and you simply can't assume that you'll be at the top. Before you commit yourself to law school, understand that there is a very good chance you will not get hired at a big firm.
I'm not sure what (if any) effect your immigration status would have on getting hired, but it may complicate the matter further. I'd ask other foreign law graduates directly about their experiences, as well as the law schools themselves.
I'm seriously considering about transfer to T6 after 1L. Will any of these schools give me some advantages for transfer? I heard that ND and W&M have good academic reputation, but Washington U & GW have higher ranking, which will help?
The general rule is that you should not attend any law school which you are not prepared to graduate from. As I said above, all of the schools you've been accepted to have good reputations. Transferring to a T6, however, is very competitive. Many people attempt to transfer to higher ranked schools, and very few succeed. You will most likely need very high grades and a high class rank by the end of 1L. Like getting hired at a big firm, you need to understand that while it's possible it's easier said than done.
You've set some very difficult to attain goals for yourself. Be realistic about your chances of attaining these goals, and evaluate whether you still want to attend law school even if you don't get hired at a big firm or if you don't get to transfer.