Also remember that the wagon's stats state that while it is on the board, all lairs spawn at +2. So I read that as 11 grabbers per lair to start, unless you only count later spawns.
I just finished that Act and I used the crawlers as Grubbers. That was barely enough, but I had my Zombicide zombies out ready to fill in.
Zombicide zombies are a great idea as a stand in for other minis. And thanks for the heads up. When we first tried this quest, we didn't see the Wagon stats so had no idea that they increase the spawns by 2. This weekend we are going to give it another go, hope everything goes smoothly!