what is EU going to do if Russia invades Poland anyway? Thermonuclear war? Are you saying the whole security apparatus and international institutions are all bluff? If so, Russia would have figured out the bluff by now.
I don’t claim to know the right solution to any difficulties in international politics, but I can say the difference between the current situation and your hypothetical is that Poland is in the EU so Article 42.7 of the Lisbon treaty world apply, and is also in NATO so Article 5 of the NATO treaty would apply too.
Nuclear deterrence doesn't mean that France would retaliate with nuclear weapons to a conventional attack, but that French nuclear weapons make it unlikely, not worth it, for Russia to use its own weapons first.
That law doesn’t prevent ~200 Russian cities from being evaporated by the French nuclear arsenal, and neither world it prevent France from responding to a nuclear attack with said arsenal.
If you accept that MAD only goes one way because one side doesn't care why can't that side conquer and rule the entire world after all for each nation the others nations plight is less than existential so they ought by that reckoning accept each nations plight until it is their turn.
Even when invaded being conquered represents a lesser loss than being blown up so one would logically either preemptively accept defeat or at least withhold your nukes while the enemy grinds you down using traditional means.
It would seem vastly more rational to work together to destroy the aggressor. If you really think current leadership will nuke you rather than be defeated then remove that leadership and see if the next leadership feels the same way.
Will we allow Putin to murder Ukraine, then Finland, then Sweden, then what? How many other lives is one mans life worth?