I see. Have you tried running the game on a pure Vanilla install? You said you made a new character, and tried the quest again but with mods loaded. Maybe try a pure Vanilla character.
Yep, thats what I meant when I said "I've already done this. I've spent literally over 15 hours uninstalling and reinstalling the game (deleting everything like mods and even all my saves just to start completely fresh)."
I deleted everything, every mod, every folder, even the My Games skyrim folder etc. Played the game vanilla without anything added and the NPCs still act the same way.
Are you currently playing? If so, I assume you aren't noticing this issue?
One thing I noticed is that while playing for about 3 hours and then doing the Markarth quest, I got the bug. But then starting over fresh (zero mods, fresh install of the game, new character) I went straight to markarth to do the quest, and it all works. I continue play with that save with no mods still and end up getting this bug on the thieves guild again. In between that NPCs that were suppose to start dialong without me starting it didn't do what they were suppose to. They'd just stand there. But since I could talk to them I didn't pay it much attention and kind of annoyingly ignored it. But now with being stuck on the thieves guild quest, it's like blah.
I'm thinking if I start yet another character, play the game for a bit and then eventually play the markarth quest, it'll bug out again.
Right now I've tried the unoffical patch which didnt work although maybe I have to start yet another character. I've gone ahead and switched to the beta but that didn't work with my current save either. I'll probably start a new character with the current beta update but if I keep running into the problem I'm just going to give up because this is way too much trouble and needlessly stressful x_x