Russia hasn't blown Kiev with a nuke because the consequences would be severe, Including possible tactical nuclear reciprocity, and becoming a global pariah who loses the few allies they have left. It also makes no sense to nuke the prize you want to own or pop off nukes upwind of your territory as your own people will be very pissed off with any fallout. It would also trigger a massive change in posture of NATO. Any country so irresponsible with nuclear weapons to use them on non threats, non-nuke countries becomes a candidate for a capitulating first strike.

Russia has updated their nuclear doctrine so NATO powers can't conventionally cripple russia by proxy via Ukraine as it would risk devastating escalation. But again, no one wants escalation. It's an unnecessary posture as Russia started this as an agressor, and played the great game very, very badly due to internal corruption. They could stop it at any time.

Edit: As for Canadian defense against the US, there is no hot war defense possible. There are 2 effective defensive possibilities.

1. Nuclear deterrent. (A sub based nuclear second strike only capability similar to the UK.)

2. Preparation for an insurgency. Like Vietnam, Afghanistan, the US can take and hold whatever it wants for as long as it wants at a high cost. Canada could not defend against any hot war. An insurgency however would make it way too costly for the US to hold onto Canada for any length of time and would be devastating to onshore infrastructure and industrial capacity at a time where China is quickly rising. They would also be expelled from NATO and would have to counter China alone without its Western World allied military capabilities, supply chains or soft power.