I don't know about the first question, but regarding the number two, you can do this. I have a .com.br account and I have already downloaded free ebooks from .com and .fr; then I think it is possible.
You will need a valid address at the United States that you will use to register your current Amazon account in the .com site. After you change to .com site, you will only be able to buy things in this site, unless you change back to .ca.
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