The USA will respond, that much is certain. But HOW do they respond and deter future use of tactical nukes, while also not further sparking WWIII? I think overwhelming conventional response is the logical answer. We are ALREADY providing weapons that are sinking Russian ships and planes. No question we would be doing alot more of that after a nuke.

You might argue that they USA will remain indirect, and just provide weapons to Ukraine.

I doubt it. I think the USA would start directly attacking, but I agree it would remain OFF of Russian soil. That would be a needless escalation.

And I think a decapitation strike might happen too. With Putin gone, there would be a fight for power, and many rich Russians would love to see all of this forgotten!

So you are right that the USA would be gone if Russia went all out. And so would Russia. Each side will be reluctant to escalate, and both sides will be sure to respond in order to maintain the "assured" part of MAD.