My only complaint is the cart being "lost" off the end of the final stage. Sure, it's a funny mechanic, it's original, and looks neat, but...
I just don't think it'll fly when it comes to public play.
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At the very least, the third stage should (like the others) have some set-up time for people to orient themselves. You could place the bombs much further ahead and have them roll back so that they're at their starting postitions when the doors open.
Edit: Suggestion: When players are near to falling into the bottomless pit on stage 3, you can put a trigger that will change their fog-controller to one that ensures anything more than X units below them is fogged to black. You'll still want the existing semi-transparent slices for long distances, but it might make the "deepness" effect more detailed up-close.