You don't have to JOIN the thieves guild for fulfilling your mainquest, just collaborate with that filthy rat to incriminate someone and the thief will point you down to the ratway, saying the old man lives there and pay huge money for not telling people he's there (wasted money). I was skiped Q from that tief guy on a market and went strait in a sewer to find Old dude. but , unfortunately, he don't have a voice! and a subtitles apeared only for 1 SEC! (two long sentence or more...) I dont have a clue what he was said. wierd! so I tried to procede like that to eventaly finished Q, but old frick stuck in High Hrothgar on peace agriment meeting. I try again, but the same thing. Many people experienced that bug, its terrible because its one of the most important (if not the most) quest in game, you should load your first auto-saved game for riften after you got "find esbern" subquest, co-operate with that damn thief and everything will work, that bug killed the experience of too much people. If you ignore my words and follow the tricks of purgatory-section one internet, you'll get Esbern bugged forever, I can tell because I tried to fix esbern with commands and nothing at all, just managed to see him... Loading previous savegames and helping scum-thieves looks like the only way.