Hummm...
1) Is there even a way to gib the zombie like that? I guess other addons would have that issue too, then.
2) To be honest, I don't know what you mean.
3) That bridge has two linedefs. The first one lowers the side walls of the bridge while the second is supposed to lower the area in front of the door leading to the exit room, simulating a "collapse" (also explains the zigzag lines on the floor - that's where it's supposed to "break"). Anyway, seems that only the first linedef actually triggers, both in the original map and my edited one. The problem: What happens to the collapsing sector depends on its neighbor sectors. If you cross the second linedef before the side walls have lowered completely, nothing will happen. This is a general flaw with that bridge.
I have solved this issue now by adding a small crack sector right next to the existing one which has the same level as the nuke floor surrounding the bridge. It's basically like a gap. This way, the side walls are not the lowest neighbor sectors any more and you don't depend on them lowering completely first. I have also moved the first linedef closer to the entrance so the walls lower a bit sooner before you reach the end. This whole thing is actually a kinda important fix since you are not supposed to reach the exit room by passing the bridge.
Fix for bridge not collapsing in MAP09 of "Treasure Hunt":Download Treasure Hunt v1.02 (1.7 MB)In the meantime,
I have updated "The Portal" again, see my edited post above. Now the pistol is replaced and the chaingun at least has a different sound, that should be good enough. ^^