I can add something to 1 and 3. For #1, in co-op, I had my buddy engage Grutilida in conversation to keep her busy, then I put a wall of ice between her, me, and the torch you pull for the cellar. She couldn't see me and I went down, looted, and teleported back to my buddy who had since left. For #3, you don't have to bash down the gate.
There are three levers nearby, one right in front of the big statue that is so dark it's easy to miss, and one on the right and left sides beyond the foliage. Pull each one and the metal door opens.
