All that blob of text is pointless. NATO need merely declare it has no offensive ambitions. Which it doesn’t. Then there’s no existential threat.
Whether Russia chooses to use nukes, likely against kyiv, is really independent of that distinction. Whether the west responds with nukes is all but assuredly “no”.
Let's examine that from a counterfactual standpoint. Let's say that during the invasion of Iraq Russia had decided to impose a no fly zone and then proceeded to shoot down American aircraft when they violated it. They of course declared that they have no offensive ambitions, they're simply helping to defend a sovereign nation against a larger aggressor. Would that have caused the United States to attack Russian forces?
Yet the bully also knows if they actually use the gun they'll be put down like a feral dog before they have time to reload.