Thankyou so much for your help! You've given me a lot to think about.
Mafia win condition is: "eliminate all other anti-town factions and achieve permanent control of the vote"
So 1v1 is not control of the vote....
The thing is, my tiebreaker rules result in all tied people having a 50% chance of being killed (independently, so both can die or neither can die). So 1v1 means that if there's a tie, they could both die or neither could die.
It's in town's best interest to vote for the scum because it gives them a 25-50% chance of winning. It's in scum's best interest for perpetual NLs (resulting in scum victory once bulletproof vest is used up), but given town will be voting scum's best interest is to vote so it goes into the RNG, 25% chance NL and we go down to the next paragraph (result: 50% scum, 25% town, 25% draw), 25% chance scum dies and town wins, 25% town dies and scum wins, 25% both die and it's a draw.
If there's NL, scum uses up one bulletproof slot, and we have 1v1 normal town v scum, however we still have the same tiebreaker rules. We have 25% chance NL and scum wins, 25% chance scum L town wins, 25% chance town L scum wins, 25% both die and draw.
So overall probability:
25% NL: 12.5% scum, 6.25% town, 6.25% draw
--> 37.5% scum, 31.25% town, 31.25% draw
Turns out the "flip a coin" tie mechanic IS REALLY CRAP FOR ENDGAME.
Possible solution: Add another rule: if there are only 2 players remaining, all days end in NL until someone wins.