Well, if anyone cares, I've "fixed" the sigsegv... I had to improvise a sort of culling plane for the tunnel with the waterfall, so now there's a pitch black block which you need to cross in order to reach the other side. There's nothing else I could think of. It looks like ass, but at least the level doesn't crash whenever you face the tunnel from the other end of the room. What should I do, keep the ugly but functional way, or pretty but crashy?
EDIT: To clarify, you go through a pitch black wall, then magically appear on the other side. It looks like a bad teleport job and I hate it. But I figured a guaranteed crash would be much more annoying, especially due to it happening when you deliberately explore a cranny in the level and then attempt to return to the main path.
Maybe I need some third-party testing on this.