Bullet point 4 seems to be related to changing the order of the models inside the level.
If I delete a model (not the last one listed in WED), then click on "undo" (the model appears now at the end of the list in WED) and then build the crash is gone. So the crash only appears if the models are in exact this order.

The reason why I'm not happy with this solution is that it just shifts the crash to another point of the game.

About 2 years ago I had exactly the same crash. The only difference was that the first level was a different one (but the second, the one that crashes, was the same). Then I just changed the order of the models inside the WED and was happy that it solved the problem. At least I thought so.
Now I noticed that the crash is still there, only the first level is different.

So editing the level just seems to move the crash, but doesn't fix it.