1. I agree that there is some tension between my points, but I think it's because they serve different principles. The US's position should be "we will not allow anyone to keep territorial gains and we will back countries who are invaded, militarily if necessary, to enforce that principle." But I also think as a fundamental principle of national sovereignty that a country gets to chart its own course. I do think that ceding anything to Putin is bad and dangerous. But if Ukraine says they're not willing to fight anymore, they want peace, and they're willing to give up territory to get it, that's their choice. Should we advise them that giving into/trusting Putin is a mistake? Yes (though Zelenskyy obviously knows it already). But it's their choice, not ours. That is the difference between the modern world and the old days of imperialism: we can attempt to persuade countries into doing the things we want them to do, but we should not compel them to do things against their will.
2. First of all, putting Western troops in Ukraine (even as peacekeepers) will be seen by Putin as an escalation, not a compromise. Second of all, peacekeepers in Ukraine will do little good if Putin turns to the Baltics or somewhere else next.
I don't care if Putin sees the peacekeepers as an escalation. Is he going to continue to fight? Is his military in any shape to fight NATO, even without the US? No. He has nukes. That's all.
And if Putin is willing to use his nukes as leverage to take all of Eastern Europe -- well, that's a fucking problem, isn't it, one with no clear answer. MAD was always based on the assumption that no mad person would be at the helm. Is Putin mad? Is he ready to blow up the world (and would his enablers) to achieve his ambitions? I don't know the answer to that.
Here's a test. You tell me what we should do. Putin nukes Seattle and tells us that if we retaliate, he will send the whole arsenal. Should we send our nukes? Should we blow up the world and eliminate the species? If you were president, would you do that? Really? REALLY? We need to keep up appearances that we would, but would we? If you can't acknowledge the problem here, then I don't think you're thinking seriously. It's a problem South Korea has faced, by the way, in dealing with Kim Jong Un. By and large, they have chosen economic sanctions, not a military response.
One way of addressing the issue would be to make a response automatic. I'm in favor of a (completely realistic and thought experiment only) policy by which all nations would give up their nukes and put them under the control of a limited AI system that has only one job: if any country invades another, the AI system will nuke them. Automatically. The system can't be bluffed. I bet that would deter wars, a lot more effectively than what we have now. Obviously this isn't a serious proposal for a number of reasons, but within a set of assumptions that could in theory hold but not in practice, I think it's clearly optimal.
All right -- so automatic response would be good, but infeasible. Back to the Seattle problem. You're president, Putin nukes Seattle, and you are weighing your response. Go.