I don't believe that the scenario of a nuclear confrontation is the most important if the US leaves NATO.
What is certain is that NATO's deterrent will become less powerful, but that doesn't mean Russia will use nuclear weapons. Since the beginning of the war in Ukraine, Russia has repeatedly used the nuclear threat, without ever carrying it out. I think it will continue in the same way, except that this threat will be more credible, at least until NATO provides an equivalent response (which is likely to be very long).
In my opinion, Russia will continue and intensify hybrid warfare to dismantle the Alliance's resistance, destabilize neighboring countries, introduce soldiers without uniforms, and then "liberate oppressed minorities" in the Baltic states or Moldova, as they did in Ukraine.
And it will be NATO's problem to determine what it is prepared to concede when facing this threat.