Just like Puerto Rico and DC?

That's why Canada would be the 51st state. Only 2 senators for all of Canada, so not much increase in Democratic representation. Representatives would of course lean Democratic, but if Trump engineers a collapse in Canada's economy, some voters may be willing to vote Republican if they're promised a bailout, given the fait accompli of annexation. Plus, if the NDP sticks around, that's fewer Democratic representatives. Sure NDP would probably vote with Democrats in most situations, but it might be easier to peel their representatives away for crucial votes.

Greenland could probably be induced to vote Republican fairly easily, and so send 2 Republican senators to Congress.

I've just spitballing, of course - in the current situation the GOP is not in the position to throw away any advantage in the House.

Only if granted statehood. There's little downside to designating them US territories. Territories don't get voting representation in Congress.

Trump's "51st state rhetoric" doesn't actually have to be what happens, were he to do something drastic.

They would never get voting rights.

Once they get to experience the freedom of the US health system they will vote republican.