Hey Greymark, the entry in Lair of the Mutant is the correct one (DEF 10 | HLT 15). Intention is for the swarm to represent many creatures so not too difficult to make contact with something but you are only taking out one bat at a time (with the 1 damage maximum as you kill an individual bat with each hit).
How far are you through reading/reviewing the books? The clarification questions have been very helpful in highlighting a few areas to edit and I will likely update the pdf's in the near future but may as well wait until you've posted all your questions first and deal with these all in one go, no pressure of course.