I was trying to get into Irkngthand without joining the thieves guild. Well, it went “meh”. First, console command to clip up to the door (when the TG quest is active, there is a bridge that is not otherwise present), then unlock the door.
After getting through to the second or third part of the dungeon, I pass a place which automatically starts the “Blindsighted” quest. Damn. Okay, I reload, noclip (tcl) past that part as well, and avoid the quest.
Approximately a hundred years later when I have fought my way through all those Falmer (damn you Vyrthur for being so aggressive, sneaking is not made easy), I make it to the final room, which is... scripted. So I’m stuck in a “scene” with Brynjolf suddenly appearing behind me. Nothing more happens.
I had looked a while ago, but apparently managed to not find the mod I was looking for. Last night then, after admitting defeat, I finally found a mod which unlocks all of these quest-specific areas, saving me from the quests and scripts.
So I will try again tonight, or whenever I have time.