Success! Using the early builder.exe makes non-crashing .lvls! Thanks LIJI, I'm looking forward to fiddling around with things.
I'd attach the .lvl file, but it's too big. I'll email it to you if you PM me your address, if you like.
I suspect the problem is that Paul's editor.exe just appends added files to the end of the .lvl. I may be completely wrong.