MAD is only the problem here to the extent one assumes that the US nuclear shield, and MAD, still protects you from nuclear escalation by Russia in the event of nuclear escalation by Russia if you are out of NATO.
Otherwise, the problem shifts from MAD to unilateral assured destruction, which is an even bigger constraint on your action against Russia’s whims.
A non-nuclear coalition faces unilateral assured destruction against a major nuclear power unless the latter is deterred from nuclear escalation by an outside force (in practice, another major nuclear power.)
> As I said, long-term, I favor complete nuke ban
Sure, and once you have accomplished and permanently secured compliance with that, your non-nuclear coalition could work for situations like the present one (but then, all existing coalitions would also be non-nuclear.)
I think all the game theory "rationality" is off the table anyways, because the very act of Ukrainian uprising against Russian invasion is already irrational (see ultimatum game). And the act of some other countries acting on Ukraine's behalf is irrational too.
MAD is just a THEORY. Very nice mathematically, but the real world is much more nuanced. And IMHO it's a scarecrow used to justify ownership of nuclear weapons, which are just horrible and should be banned.
Addendum: Hitler actually wanted to destroy Paris (see https://en.wikipedia.org/wiki/Nero_Decree), but it, almost by miracle (in reality it was brave work of different people involved) survived. We need to have a hope, but also get rid of all the nukes.
There has been some progress on this in recent years, see https://www.icanw.org/. And in fact the nuclear arsenals of USA and Russia have been (albeit too slowly) reducing over the past 30 years. But more governments (NATO expansion) relying on nukes for their security (which is a false sense) is a pretty big setback.
The USA and Russia reduced their arsenals primarily as a cost savings measure. Keeping the weapons systems operating is shockingly expensive. Modern delivery systems are more accurate so we can now ensure MAD with fewer warheads.
I am Czech and this war convinced me to be against our membership in NATO, which I think is ill-conceived, for the following reasons:
1. We should have our own defense, both on national and EU, and not rely on USA, who might have different interests, and is not always making us safer. USA might accidentally provoke Russia (their archenemy) or they might decide not to get involved in our defense for political reasons (actually, in democracy, it should be their right), leaving us exposed.
2. NATO causes proliferation of nuclear weapons, by expanding the trigger area, and also stations nukes in countries that don't own them. I favor a complete denuclearization of the world, IMHO MAD is a crazy theory that relies on rationality of people involved too much.
3. European NATO members are at this moment incapable of stronger (military) help to Ukraine, because of the nuclear (MAD) threat. NATO is now actually a reason why we cannot fight Russian invaders the same way Allies did fight against Nazi Germany.