Talking about events 16 - 18. There is no 1 solution, because there are many problems.
Lets list up the problems (those i see) , and see if you see how to fix them.
1/ There is no zombie picked in event 15.
2/ There is no system (preferable containers) that links a zombie to its ZombieArea. So, even if you picked the zombie, you can not pick its ZombieArea. Overlap condition will not help, because sometimes they are with 4 on almost the same place.
3/ Since there is no ZombieArea picked (so it is an un-referenced object), the actions addressed to ZombieArea, work on all of them. So, each one is set to solid. And there will be an obstacle region generated around each ZombieArea. You can as well regenerate the whole obstacle map, and have them always solid, if you not gonna pick them.
4/ The obstacle map is ready in the next tick. But you force them to find a path in the same tick. Before the obstacle map is ready. In event 16.
5/ And the worst. There is only 1 obstacle map. Every time you send a human out to find a path, it considers the same obstacle map. Now. If you get it to work, then every time you mark a region on the obstaclemap as en obstacle, it stays an obstacle until you clear it. So, the way you do it, in some short time each cell in the obstacle map will be marked as an obstacle. See it as stamps you set.