Lived Ukraine, Odessa and Nikolaev for a few years (2008-2010) can say with certainty that if the Russians invade, there will be country wide civil war, and it will flare up immediately and be more costly than the invasion itself.
The antipathy between Ukrainians and ethnic Russians is palpable, resulting for instance in the Odessa fires a few years back or so, viewing the YT videos of young Ukrainian women throwing petrol bombs into a building with holed up folk was eye opening even to me (and living in Eastern Europe/Russia for two decades I've seen a bit).
Many of my Ukrainian friends, civilised professional folk, had some deep seated hatred lurking in their brains that spilt out on occasion and it wasn't pretty. The Russians invading will snap some of them instantly into ugly people with some nasty intents.
Not a military genius but expect Transnistria will play some role in this as some sort of Russian flank.
Genuinely hope this rumour has no foundation.