I think there is one more flaw that has not been covered yet:
When enemies start taking different routes because towers have been placed in front of them, enemies ahead still follow the old version of the valid path, which don’t use the WillBlockPath() method in Pathfinder script anymore, as it is only used for the spawn position > destination position path.
I thought about it and tested it: while other enemies caluculated a new path due to towers placed in front of them, I succeded to isolate an enemy that was already ahead of these towers, using other towers. When this enemy was completely stuck, it simply disappeared from the map… 