There are already multiple posts on this thread about it.
It's not that easy as you make it sound like switching a flip and one value can be changed to another one or be altered into something similar.
HP of NPCs is completely different to weapon damage hence it would be a new option and remove another/don't use it.
In my experience so far, I was hitting the wrong key consistently. So double check the right key is being pressed for one.
Now if you weren't dumb like me, what I found worked when EXP not given, I'd activate the option, run around a bit, disable the option and then when I killed an Orc captain my EXP would skyrocket. That worked sometimes. Other times, I'd just leave the option turned on until I killed a captain and it'd work.
Shadow of War doesn’t use TruePlay. So feel free to disable it.