In practice, Finland and Poland are the greatest protection against Russia as they are very much in the way and would trigger all of NATO.
Many European counties have very credible defence industries. From what we have seen in Ukraine, NATO (even without the US) has both a technological and tactics advantage against Russia.
We are also seeing that warfare has changed. He who makes the most drones and cruise missiles wins.
And Europe could massively outspend Russia, again without help from the US.
Germany has massively stepped up their military spending. If they had to, Norway to completely fund a NATO war against Russia on their own.
The only thing the US really brings to NATO these days is their nuclear arsenal. But again, Russia has shown that their “superiority” in this area is useless in practice. It will not help them in a war with Europe.
The only way that Europe loses against Russia is if the US teams up with them. This used to be an insanely unlikely outcome but….
The most of countries likely to invade Canada is as follows:
Nobody else could manage a land assault on North America. Coming in from the arctic would be almost impossible.
Invading from the west or south means the US is in the way. And the US is unlikely to tolerate an invasion to the continent anyway (unless they are involved). And where is an invasion from the east being staged? Europe?