OK, I have some info. I contacted some big law firms (top 100) and grilled recruitment people about various issues. I don't know how relevant this is to Canada but for other int'l ppl it may be useful.
1) A student visa _will_ allow you to do a 2L summer in an American law firm, no probs.
2) They will apply for a visa for you to stay in the US if they want to employ you. This is not a problem, it is done many times, every year. But almost all say they wouldn't bother unless you had done your 2L summer with them. Which is good to know.
3) I contacted British and American magic circle firms in the City (London): Lovells, Linklaters, Clifford Chance etc etc. all of whom said it's not a great idea to train in a foreign jurisdiction and then take the QLTT (Qualified Lawyers' Transfer Test) to convert to English law. There will be a similar equival;ent in Canada. Though technically by taking the test you are qualified in your home country and the US, they say your knowledge would not be in-depth enough for them to want recruit you, without you having to retrain, unless you were already a very senior lawyer. Perhaps this isn't the case in Canada, but it's certainly worth checking, just to be sure. So I'd write a few emails to law firms that interest you.
It has definitely given me a lot to think about! I'm more informed, but more confused, than ever!