Hey, thanks for the time and effort you've put into making this great game. I can tell right away it's a well-thought-out map, and carefully considered game. I noticed during my first play-through, when I got to the safe place to camp in the beginning, right after having Erika join, I was attacking the practice dummies, and wound up falling through the ground, doing the long drop. No death, no escape, just falling. I had to restart the game from that, but thought I'd let you know.
Suggestion: Add code that checks for the player being below the lowest solid object on the map, and mark a point on each spot as a default location. If the person winds up below the map, drop them at the default location.
This won't solve the clipping problem from happening, in the first place but it's the industry standard practice for handling it when it happens.