I give up on trying to find enough time to reply to everything all at once so I'm just going to work my way through in bits and pieces. In order with one initial exception. If I skip over something you think is important then please raise the issue again, trying to hit the big points here.
First off, PFS rulings are garbage if you want martials to even approach the power level of a mediocre 9 level caster, stop using them.
As I mentioned in my last post, that's not my current concern here. I have plenty of knobs to adjust in terms of martials vs casters.
What I'm currently concerned about is that if someone wants to make Roy Greenhilt using a Greatsword or Gimli using a Greataxe then that shouldn't be massively inferior to using a Polearm with Shield Brace (which is a feat in a player companion and literally had to be PFS-ruled to weaken it).
I'd much rather say "Okay all martials using melee weapons get +5 AC" than say "Okay all martials who want to use melee weapons need to use Shield Brace."
It sounds like you aught to have paired some feats together rather than giving your players extra feats without limiting their selection.
I mean, their selection is limited per the approved content (CRB/APG/ACG). I figured they'd run out of "focused" feats and need to diversify which was the goal.
But yes, a much more involved solution would have been potentially better. Runs into problems of its own, though.
The reason why you'd expect a stat difference between a dual wielder and a non-dual wielder is that the single weapon fighter doesn't need the dexterity to qualify for dual wielding. This doesn't always matter if you aren't using point buy, or are using too high a point buy.
Point buy rules are nothing above an 18 or below an 8 pre-racial, 25 point buy. Used to be <= 8 and >= 7 with 20 point buy but I had some players who wanted all their stats reasonably high and were hurting the party. So I agreed to give them a slightly higher point but limited dump stats (someone who had two 7s only effectively gets a 21 point buy).
Also I would expect most Fighters to want at least 14 Dex for Armor Training. Which means you just need 2 more point buy points to get to 15. At that point you'd wear a belt to get to the higher Dex requirements (from my understanding of what people actually did).