The setup could easily be varied in a lot of different ways including player count, but let's start here:
Nightkills are mandatory, probably. Each vanilla townie is, in their role PM, told the identity of one other player, and informed that they are also a vanilla townie. This is done in a circular fashion so that all of the vanilla townies form a single loop. The mafia win normally if they reach parity. If the town eliminates both mafia, then the mafia's alternate win condition mechanic activates: they pick a player as the "start" of the loop, and PM the moderator a guess at the order of the loop starting from that player. If they make any mistakes, the mod tells them where the first mistake was, and they get a second chance. If they fail that, the town wins.
Example: the town are labeled A-G and their loop is in that order (A -> B -> C -> D -> E -> F -> G -> A). The mafia guess the chain E -> F -> C -> B -> G -> A -> D -> E. The moderator tells them that the chain is correct up to F. They then guess E -> F -> G -> A -> D -> C -> B -> E; this is still incorrect, so the town wins.
Easiest way to change balance in principle is removing the mafia's 2nd chance or adding more chances.
If the town doesn't use their information at all and pretends the game is mountainous, they'll be at a severe disadvantage (7:2 mountainous is 30% EV). If they openly massclaim, then they'll be able to determine exactly who the scum are but the scum will win automatically. So it's about finding a happy medium between not taking advantage of the setup and being too open with information.
One obvious strategy is that before hammering, if a player can claim the hammeree as their clear, they do so. However, doing this more than a couple of times has a lot of risk, and mafia could interfere with it by having one mafia claim the other as their clear; if town then refuses to carry through with an elimination it could be tricky to get back on track.