Мафия 6. The Will of Swamps

by the way, I wanted to know... if the Necromancer visits Mafa and turns him into an idiot, does he automatically switch to the town's side, or can he still help the Mafia? Because it's impossible to tell whether he's communicating with his accomplices or not.

No, he continues to help the Mafia, meaning he can witness the Abbot's murder or the Rune Priest's murder and, therefore, hint at it or convince them to discuss it – something like that.
P.S. If he switched to the town's side, it would be funny: how could he forget about all his accomplices...