Thanks for the feedback! I realized there were some missing connections on my map and the routes in/out weren't marked, which I think reduces the linearity. I also had cut the description of the exterior which hints at the second entrance, which I've now added back in. I do agree though, it turned out more linear than I wanted / expected.
On the magma level rising, it will never actually inundate level 3, although I can see where the confusion comes from. I added some notes to the delving rules section to hopefully clarify.
Looking forward to reading your entry!