Yeah we definitely didn't sign post some of the features/gameplay properly, I imagine most people would have missed the mutation side of things.
The zombie spawning still needs balancing - we thought one hit kills for the first set of zombies would be enough to sort out how many there are, but the time between spawns probably needs to be slower as well. I like your idea of nudging the player to kill zombies through more definitive feedback; we kind of assumed that would be the first thing people do, but then the results aren't always clear or satisfying enough to continue with it. We'll definitely be giving it some thought!