So the trick to getting rid of immortal Zamasu will be his fusion with Goku-Zamasu.
Since the earing fusion is absolute, can't be undone and Goku-Zamasu isn't immortal, killing the fusion between immortal Zamasu and Goku-Zamasu will also kill immortal Zamasu.

I do not see Mafuba playing a role. It's long in the past now, but I think I remember Mafuba wasn't very effective against an oponent that would be considered extremely weak and slow by current time DB universe. You could always argue that the technique's power also depends on the caster's powers. But since it was a bit slow to cast and act, and since the pot is so fragile and abvious target when the casting is starting, I do not think it can work against an oponent who is at least as fast as you are and probably as strong too... But more importantly a lot more cuning and intelligent.