hm, tried your save and he attacks the Lost Ones as soon as he's finished with the highwaymen
- don't attack, retreat back to the door when they approach,
that way he should have enough time to make sure the bodies are indeed dead
(just for testing purposes, maybe try to wait upstairs with godmode),
or, better,
- (load an earlier save and) come back in like 10+ levels (the area is doable with your level, but probably not much fun

)
if he
really doesn't do anything on your side,try to
check the game files
edit:
tried again, he really seems to have some issues when the player is in combat :/
sneak down the stairs, wait for him to finish, hope for the best.
I' ll open a ticket in the bugtracker
