As described, I think those rationales have a little strawman quality to them.I've been reading two rationales for why the United States shouldn't act. One is that the Russians have nukes and we don't want to risk a nuclear exchange that would end humanity. The other is that Ukraine is not a member of NATO, so we have no obligation to help them. If you think that the threat of nuclear war is too high for us to act in Ukraine, then why would Russia invading Poland or one of the Baltic states compel us to act? Just because we signed a treaty, we're willing to risk the nuclear scenario? If the nuclear card is the end-all, be-all of foreign policy, then agreements like NATO really mean nothing, and we can't live in a world like that. That means the nuclear threat is a red herring in determining our course of action. What's left is to decide to do the morally right thing, even in the absence of treaty obligations, and act to the best of our ability to protect the lives and rights of the Ukrainian people.
1. Re: nuclear, yes, that's the elephant in the room, but it's really a baby elephant, and it feeds into the strategy of not escalating things. In other words, it's not just about minimizing nuclear escalation risk, short of Putin "literally" being a madman, which serious people don't give too much credence to. It's about preserving room for de-escalation, i.e., the exit strategy, graceful or otherwise. While it may well be that as a practical matter a "truly neutral" ukraine is no longer realistic going forward, a "less armed" Ukraine (eg, with a third party security guarantor) might still be a viable pathway out.
2. I don't think we're not helping UKR more simply because we're not obligated to. Mainly because, we are, in fact, helping them, a lot. Rather the lack of an obligation (and maintaining that distinction) reinforces the nonescalation/minimize escalation/no threat to russia message that has been at the heart of our approach. Tactics should always yield to strategy.