I would REALLY appreciate it if anyone can provide some insight into the actual meaning of the hero growth stats. I have tried comparing hero stats before and after fusing as well as before and after promoting, and am still at a loss as to their significance. While in some instances I could see a small bit of correlation, most left me baffled.
I’m pretty good at math and tried many variations of percentages and multiples, applying them to both the base ability rating pre-promotion/fusing, for example, the combined rating with equipment, the equipment rating alone – nothing added up!!
I was hoping that understanding how these work might help me choose which heroes with similar abilities to develop. Thanks for any help. It’s making me crazier than I already am!!

You and me both. Unfortunately many stats and the definitions of the abilities are very vague, and asking the devs does not always end up with an answer. Make the best choices you can with what info there is. This is a RPG after all.