This series of quests can have problems if you do things too much out of the expected order. However, you do not need to talk to Trevor before entering the sewers. I had Tingalf arrested before I even went to Verdistis; when Trevor asked me to look into a suspected black market dealer, I could tell him right away that he had been deal with, which completed that final portion of the quest.
After you enter the sewers, you should be able to defeat Pedro (just to the right of the entrance), retrieve a letter he drops and then go speak to Lr Robin (usually north west corner of the Ars Magicana square). At what point do you have a problem?
Marcus-Tingalf-Trevor-Quest