It wouldn't be for hostile purposes. If Canada wanted to build nukes as part of its general defense strategy as a sovereign nation could they? They wouldn't be aimed at the US (at least for now -lol). Can Japan start building its own nukes? why not Canada.
When you were asked “Why” Canada should acquire nukes you responded with:
The Bully south of the Border and the Republican Party. We look/are weak without them. Why doesn't Canada have Nukes?
How would you think we could get away with something as duplicitous as this? Telling our friends and neighbors to our South “Don’t worry, they’re not for you (at least for now - lol)” might not be entirely believable, especially since there would need to be a reason less asinine than the one you gave (“The bully south of the Border and the Republican Party.”) That poses an unacceptable risk, particularly in knowing that the success of a full-scale invasion from the South would be measured in hours, if not days.
As you know, isolationist policies (DPRK and Iran) or a regime-change (Iraq and Libya) would NEED to be implemented on the grounds of national security interests.
Japan is a de-facto nuclear state. It likely has the ability to produce a weapon within a year of changing its stance towards the NPT. Why it hasn’t is for similar but different reasons from the Canada; it would negatively, perhaps irreversibly affect its relationship with the USA. As a hegemonic power, even allied states that dramatically disrupt regional dynamics in SEA and EA, pose a risk. Particularly if whatever Trumpty agreed to includes complete denuclearization of the Korean Peninsula.
Yes, Canada is a sovereign country and has the legal right to produce weapons it sees fit in-order to protect itself (once they pull out of the NPT - which we won’t). It would be the stupidest move in the history of defence, as the country would be long-gone by the time the program could actually be completed, as US security needs trump rights to national sovereignty of hostile (and pretend-friendly) nations every time.