The Russia collapse scenario I always found fanciful.
If the deal is freezing borders (more or less, certainly no more territorial concessions by Ukraine), Ukraine does not recognize Russian conquests, European peacekeepers(probably EU + UK), and eventual admission to the EU, no NATO admission, I think thats about the best Ukraine could realistically hope for in the short term.
If they join EU, EU also has mutual defense pact, so they wouldn't need NATO.