Hell some US brokers won't take Canadian clients. Some kind of cross border agreement. I don't think Canadian brokers will take US clients. (Stock Brokers, not sure about futures or forex)
I have no idea. All I know is that years ago, before Canada had discount brokers I opened accounts with several US discounters. They were eventually closed due to whatever regulations.And that's what I see as potential problem "some sort of agreement". Between whom? Based on what regulation issued by which regulatory body?
How many of them offer AMP levels of leverage (for day trading)? Low intra day margin requirements.
IB was wanting something silly like $20,000 for day trading one ES lot a few years ago, this made IB unusable to me for day trading. So I left and opened an account at AMP.