How to play Werewolf/Mafia with exactly 6 players?
I've found one option I overlooked until after I asked this question.

It appears that the "good" side has a 44.4% chance of winning if all votes are random, they have the option to not lynch someone on a particular day, and there is only 1 mafioso. A large portion of this, however, relies on the 16.7% chance they successfully guess the mafioso at their first vote. This would make it more of a "serial killer" game.  This is far better than I initially thought.

I suspect it might be most fun if players are told they only win if they survive, multiple rounds are played, and they can lynch as many players as they like on a particular day as long as each defendant received a quorum of the votes as required in the original mafia game.

This answer is only speculative and, therefore, not a good answer.