I don't think the ball thing is much of an advantage, some QBs like them lower pressure, some like them higher pressure. You can intentionally injure someone and only get a fine or one game suspension.
And? I don't give a shit if one QB likes it overinflated, and another likes it underinflated. The point is, Brady obviously likes them underinflated, and it is OUTSIDE THE PARAMETERS SET BY THE RULES. And yeah, if you can get a better grip on a ball, especially a cold or wet one, it is obviously an advantage. And has been stated by numerous players, not just QBs. Not sure why this is so hard for you to understand.
And intentionally injuring somebody is a whole other thing and has absolutely nothing to do with this. Hey, did you know that sometimes defenses try to mimic the offenses cadence to try and fuck them up? Yeah, that has absolutely nothing to do with this, either.