Is it possible to get a medical certificate from Canada and a pilot license in America?
(This is similar to this question - and see this answer in particular - but there may be some points here that deserve a separate answer.)

First, can you use a Canadian medical instead of a US medical, with a US pilot's certificate? The answer is no, with one (?) exception. If it were possible, then people could shop around other countries to find the easiest medical for them, which would be a huge loophole and would allow them to simply ignore the FAA's medical requirements. As a general statement, the country of your pilot's certificate and medical certificate must be the same.

The only exception I'm aware of is when you hold a foreign license and medical, and then ask the FAA to issue a foreign-based private certificate under 14 CFR 61.75. See this question for more information but basically, you would have to hold a Canadian pilot's license and medical first, and even then you would be limited by the privileges of the Canadian license.

Second, could you get a US commercial certificate using a Canadian medical? Yes, but only in a very roundabout way, and even then you couldn't use it. You'd first have to get an FAA foreign-based private certificate as I described above - which means getting a Canadian license first - and that certificate would be fine to use for a US commercial checkride. However, after passing the checkride you'd still need an FAA second-class medical to actually exercise your new commercial privileges.

Finally, another option would be to get a Canadian commercial license and medical, and then fly only Canadian aircraft in the US. I have no idea how practical that is because most countries don't allow cabotage, but maybe there's some way to make it work.

AFAIK diabetes is only disqualifying if you're insulin-dependent; if you're on oral meds you may still be able to get a Special Issuance for it. If you have any doubts about your personal situation then I'd suggest consulting an AME and/or calling AOPA, if you're a member.