With how easy it is for Masons to win the final objective, I think the best way to handle the final objective would be to allow players to defend the surviving garrison as an armed militia with weaker weapons than everyone else, but is still much stronger than regular peasants. Perhaps this could be how the armed militia could be balanced:
Base class: Beginner Mode Bot
Slot 1: Shovel (Field Engineer)
Slot 2: None
Slot 3: Knife
Slot 4: ?????
New ability (specific to Militia): ?????
Recharge rate: Very slow (worth 3 bandage kits at full meter)
Pulls out a chicken out of nowhere. Under normal circumstances, a player controlling a militiaman will never build up enough meter to see what their ability will do.
Hypothesis: Coxwell peasants would still be grossly underequipped to fight off the Mason attackers, though the having of an extra 10 points of health along with the Field Engineer's shovel and the standard kitchen knife as weapons could allow individual player skill to overcome the disadvantages inherent with spawning as a peasant. Also, having an armed militia in Coxwell would provide players an opportunity to play as those bots you've killed hundreds of in Beginner Mode.