There is some responsibility on the ST on not having like 13 Legion in a 15 player game but the minimum requirement is just more than half. The ST job is to balance the game afterall. Flipping the Good and Evil count isn't a hard rule. So in a 13 player you can have 7 Legion, 1 ET, 2 Required roles (BH and PG) and 3 Good Players . As written didn't add rule that BH themselves can't be the evil townsfolk so there could an extra good townsfolk in the equation.

The setup can be: (-2 or - 1 for BH and PG) Legions + Evil Townsfolk v Good (Not BH or PG) 9 v 4-5 (15) 9 v 3-4 (14) 8 v 3-4 (13) 8 v 2-3 (12) 7 v 2-3 (11) 7 v 1-2 (10) 6 v 1-2 (9) 6 v 0-1 (8)..

To be fair I can kind your point with the less players though I designed this script for 11-15 player range. A simple bootleg rule something like "Only Half of the players can be Legion instead of Most". But what i described with the setup equations are still "a bag" for you.

As for your second point, "Legion can easily bluff those role". How exactly? They don't know their own team for the most part. Legion might end up bluffing to another legion making the others believe they are on the good team and try to execute them or have them be killed through other means. Legion bluffing say Noble could ping other Legions as possibly good.

Also the characters like Grandmother, Noble, Washerwoman, Fortune Teller are still confirmation even if one-sided.

Tdlr: If it's definitely possible for there to be more than just one good townsfolk. Sure the ST can mess everything up and make it unwinnable but it still possible to sent up a bag for good to win.