The mold concern I have is not in the attic it is concern for mold growing in the DUCTS. Since the heat inside the ducts seems to be swinging from 90+ (or more) in between cycles and then back down to about 55-60 degree range, that swing is probably going to cause mold, at least is what I'm thinking based on what I've seen in Florida. Such a quick temp differential causes moisture build-up, and being in a dark place like ducts it will then fester and grow.
I think you'll find that you need LIQUID WATER aand Food ( dirt) to grow BUGS ( mold) and the temp range is not the controlling factor
You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!