Canada

Yes absolutely, any change to the monarchy needs a constitutional amendment, which basically means “not gonna happen” because of everything you outlined, unless there’s a massive shift in opinion among Canadians.

The best short term outcome here is probably for the Canadian government and courts to just conveniently ignore or bend some rules… let these people swear an oath of allegiance to the constitution and let a judge do some hand wavy legal magic and say “hmm, close enough”.