This has never happened to me before. I'm on PC, no mods except the unofficial patch (which I've always used (and I don't really consider to be a true mod anyway)).

Went to Geirmund's Hall (GH) as part of the Forbidden Legend quest. Wiped the place clean including Sigdis Gauldurson. The dungeon didn't read as cleared, nor did it add to my count of cleared dungeons. I figured, "fine," sometimes you need to reclear a dungeon that is quest-related, so maybe that's the issue.
I returned to the dungeon over 100 days later and it hadn't respawned. Sigdis' corpse was still there, too.

I have tried the "setlocationcleared" command, but it doesn't recognize the cell ID (even though I'm using the ID obtained via the "help geirmundshall 0" command, which retrieves the same information I confirmed in the Creation Kit (CK)).
I have tried resurrecting and re-killing Sigdis (and also disabling/enabling, too), but when he respawns he is unkillable - he simply goes into that recovery mode thing, and then snaps out of it (non-hostile, too).
I also tried creating a simple mod that sets the cell respawn rates for locations (clearable and unclearable) to 12 hours, left GH, slept in Solitude for 24 hours, and then traveled back - and the cell still doesn't respawn and the aforementioned console commands still don't work.

Any ideas (to clear/respawn the cell)?

Thanks.