Getting into Canadian med schools as an OOP (out of province) applicant is stupidly hard, as an international applicant it would be just as bad or worse. Canadian students apply in the US because there are so few Canadian schools that you need a way to increase your chances in a given year. There's very little incentive for US students to apply in Canada, as there are so many med schools in the US that the cost of applying to all of them will limit you, not the number of schools. You could apply if you want, but there are so few seats reserved even for OOP Canadians that your chances would be vanishingly small.
As for your other questions, getting a US MD means that you're still relatively competitive for matching in most Canadian specialities, but Canadian MDs will still get priority. Going to med school in any other country or getting a US DO means that you are considered an IMG, which means that you'll get into Family, Psychiatry and sometimes Internal. Everything else will be really hard, and you'll probably need some sort of connection to get a spot. I'm not sure how Canadian MDs applying for US residencies fair.