@csamba
I have a feeling that your earlier ways to get in have been unintentional glitches. The easiest/least failure prone way to get in is to go out of the window in Thereon's guard's room, then go along the ledge, then either chance a jump over to Thereon's window or use a rope on the roof beam.
@Squadarofl
To help the mage go to the upper floor of the Stumbled Traveler and talk to Thereon. Just watch out for Leautenant Ashley patrolling the lower floors.
@Marbrien
Garrett saying that he has no reason to go to the Raven's Nest yet isn't misleading. Once the player has found a reason to go there he says a different line about needing something to get back over the fence, making the stool frobbable. Have you listened in to all the conversations, including the bunch of guards at the House of Science barricade? They talk about the Raven's Nest, irc. Or maybe try pickpocketing one of the guards with scrolls cases on their waists for their orders.