Soljund's Sinkhole is an active mine, but the men refuse to work there because . Perth is outside [censored]ing and moaning about how the Jarl is too lazy to assist. I believe I killed every enemy inside, but Perth does not acknowledge this and the mine is not tagged as clear. Went back in and saw nothing. Was I not supposed to go in there, perhaps it was a quest for later? Perth's dialogue allowed me to declare that "I go where I want, Stormcloak collaborator scumbag."

I kinda "accidentally" murdered those guys, but it didn't clear for me either.

I've been unable to clear Fort Dunstad and Fort Hawk... something near Morthal. I suspect they have quests attached that makes them unclearable until they are completed. Was the case for Fellglow Keep atleast, there is an area that is unenterable if you're not on the quest.

Those have quests associated. Dunno about the sinkhole.