I can see the lever in question, though I'm slightly at a loss as to how to get to it without being incinerated, frustratingly. Fire-resistance scrolls don't work on lava, apparently telekinesis doesn't let me grab the lever even when standing just a few feet away, and dropping things on the traps doesn't get rid of the lava. I discovered the sentinel isn't invulnerable, but destroying it didn't seem to accomplish anything useful, so I seem to be at an impasse...