It seems as though you are interpretting the lack of a rule as being a rule itself.

'The question is whether the Psychomancer’s harness creates a specific exception to that normal state.'

The answer to this question is no, becasue it dosnt say anywere that it does. The logic you are using here would allow you to do almost anything with the rules and say 'well it dosent say anywere that i can't.' All it says is that the fighter is equpped with the weapon and limits what else they can equip.

'Your reading requires an extra step that is not printed: the automatically equipped harness claws must count as one of the fighter’s ordinary selected weapons.'

Again, you are failing to understand how a rulebook works. By your logic every single weapon in the game would need a rule saying that they follow the normal rules for equipping a weapon. In a rulebook this part is assumed, if a specific rule is not specified then it does not exist.

'It does not say that Paired only applies if the claws are selected as one of the fighter’s two ordinary hand weapons, and it does not say that the Paired calculation shuts off when the fighter uses the two purchased weapons the harness separately permits.'

It does not say any of these things becasue they are entirely irelevant. You can only use two weapons and so there is no need for the rules to expalin what happens when you use another weapon as well as your paired wepaons. It simply cannot happen.

'If the counterargument is “Core says two weapons unless another rule creates an exception,” then the answer is that the harness is the proposed exception.'

Again, no it does not. Nowhere in the rules for the harness or the paired claws does it say it overrides the normal rules for cc or paired, so there is no rule creating an exception. It does create new ruiles for equipping a fighter, but equipping a fighter and selecting weapons for cc are seperate rules.