"plura mala" actually means "more bad things" (literally), but I translated it by "worse harm", because it is the best interpretation of the words here I believe.
When I compare our versions I can see a crucial diffrence, dear. From yours "The demon couldn't cause much harm to the clergy, except for assigning to it reckless serves." I could deduce that the demon cannot cause much harm! Meanwile he is able a lot! And the worse harm what he can do is just giving negligent servants to the clergy.
Yes, not an easy task when translating... Sorry for my English.