I don't like presenting things as black and white but I also feel the same - if US is no longer part of NATO we should expect them to be in opposition to it.
In a conventional fight Russian forces would be wiped off the Ukrainian map in less than a week. The problem is that nobody dares to engage them now that they are at their absolute weakest. The situation might look very different depending on how much time they get to rearm and the stance that the Americans will take.
I don't think russia would use nukes. That would open them up to retaliatory nuking. France and Britain both have nukes and the russian border is a lot more porous and a lot longer than the european ones. Much easier to get a nuke to moscow than to london.