“If the Ukrainian government breaks down....do they risk the Russians moving in and taking over the entire country”
It would take some real breakdown of society for that to happen, I think. Russian money has subsidized Ukraine for two decades. IMO, Putin is more than happy to pawn off the corrupt and destitute Western Ukraine off on Europe. The real wealth is in the embattled areas of the East.
“do the Poles decide to take action and move into Western Ukraine to stabilize things there?”
It is far more likely that the Hungarians would move into Ruthenia, than the Poles would do anything.
The Hungarians have been fairly vocal about the forced conscription of Ethnic Hungarians. There has been protests and some unrest in that area in recent weeks. I posted an article about it some weeks back.
Heck the Hungarians would take all of Slovakia if they could.