Then there will be a hot nuclear war.

No something I welcome, but the calculations you are making don’t match up with reality.

How would you have it? Just let any nuclear armed state invade any country it wants? There has to be a place where the line is drawn.

I have trouble understanding your point of view. What are you suggesting? I hope not that US starts nuclear war with Russia over disagreement about Ukraine?

In the case that Russia drops a nuke on NATO troops, you believe that the US should just accept that with no response?

Russia has agency in this scenario. If they drop nukes first they are starting a nuclear war.