Q&A for How to Infiltrate Mercer's House in Skyrim

This definitely isn't supposed to work like this, but here's how I did it. I jumped on the fire posts out in front of the Temple, jumped onto the roof, and shimmied my way up as high as I could go. (A tip for this part: strafe around the corner of the roof, spamming space bar. You should slowly be able to get high enough.) Then, I positioned myself to face the porch of the Manor, and used Whirlwind Sprint to cross the gap. I'd recommend dropping a quicksave in case you miss; it saves you having to go through the trouble of climbing the roof again.

To get his house key, you need to kill or pickpocket Vald, Mercer's house guard in the courtyard of Riftweald Manor. Vald has the key in his inventory, which will allow you to open the manor door after shooting down the staircase (the manor door at ground level is blocked off and cannot be opened).

Presumably, you're in the quest "The Pursuit". In your Quests tab, set this quest as a priority, which will give you a compass for direction. Once you break into Mercer's house, the compass will guide you to a room behind a "Suspicious Cupboard/Dresser". You will then steal Mercer's personal notes and then go back to the Thieves Guild, where you will meet with Karliah and Brynjolf. These notes prove Mercer's betrayal. They will go through some dialogue, the rest of the guild will turn against Mercer, and a new quest to become a Nightingale (after which is a quest to defeat Mercer) will start.

No, he's not. Only two low-level bandits await. Other than that, all you have to do is find the suspicious cabinet and pass through some traps.