Your video inspired update 4, which is uploading now. I improved the collision on the vehicles. I also increased the ambient light to make it a bit brighter, and I put lights on the weapons and items to make them easier to find. I made the second characters level much brighter too to make that one even easier.
The soldiers have like 3-4 times as much health as some of the zombies which is probably too much, i was figuring because they would have bullet proof vests it made sense, but overall everything is too strong, i'd rather make them weaker than less of them, its nice to have as many zombies as possible.