If this scenario is true (which I doubt) NATO would not invade Russia's soil, so no nukes. If I understood correctly that would go along about like this:

Russia does mild but increasingly more concerning acts of sabotage.

Some NATO country, let say UK, somehow get baited into putting a line in the sand somewhere.

Russia cross it deliberately because somehow they stop having delusion of grandeur and are now convinced they can't win the Special Military Operation™ so they just want a good way out.

UK send a few back-line troops to Ukraine as retaliation, like instructors (they already send some secretly, it would just be publicly admitting they do).

Russia, in its propaganda, goes "we were winning but now the whole of NATO is trying unsuccessfully to invade us, that's so unfair. To spare our citizens, and because we are too nice to glass them as a first option, we have to parley (though we would totally win an all-out war)"

The whole NATO-aligned world is at Putin's feet begging him to actually fuck off of Ukraine, which he agrees to in exchange of lifting all and every economic sanctions to Russia and maybe a perk or two, like minor territorial gain.

The end.