Off Hand Only Why?

[Motenten]

You actually have 3 rates and 2 composition factors.

The proc rate of the weapon. Always 30% for all weapons at 100 TP.

The number of base attacks possible per round.
2-handed weapons: always 1
H2H: always 2
1-handed weapons: usually 2 because of DW, but can be 1

So in 100 attack rounds, the number of ODD procs would be:
2-handed weapon: 30
H2H: 15
1-handed weapon: 15 or 30, depending DW

Since 1-handed weapons are -usually- DW'd, you can generally expect that, for example, Kannagi will get the same proc rate as Verethragna, both of which are half the proc rate of Ukon.

Given the relative strengths of the weapons, I would be inclined to suggest a 50% increase in ODD proc rates for 1-handed and H2H weapons (ie: 45% proc rate at 100 TP instead of 30%, up to 75% proc rate at 300 TP instead of 50%). That would still leave them weaker than 2-handed weapons in terms of ODD procs, but helps make up for the fact that they only get half as many chances per round. And while single-wielding 1-handed weapons then gets them a significant increases in relative ODD rates, they'd also be giving up the rather significant advantages of Dual Wield, so the gain (if any) is not nearly as large as it might at first seem.

[Diasetsu]

It's not just Empyrian and Mythic H2H weapons, Spharai is the same way, OTD is only for Main Hand. They changed ToM OAT H2H probably because you can dual wield OAT Magian weapons along with a kc, ridill, or other Occaionaly Attacks multiple times weapon. Now since H2H has always had a pseudo-dualwield effect but no OAT knuckles prior to this(we'll get to mythic in a second), they never worried about it. Every dual wielder can have weapon A with OAT and weapon B with OAT, so you have to allow both fist A and Fist B to have the OAT to make it fair.

Now why Mythic/Empyrian/Relic only proc on one hand you only have to look at balancing towards dual wield just like they did with ToM. In all cases of any of the 3 "Ultimate" Weapons when in the sub slot, or weapon B as we'll call it, none of these bonuses apply. You don't get 2x, or 3x Relic procs while off handing, you don't get Aftermath on offhand Empyrian or Mythics. H2H Ultimates only allow 1 hand to proc bonuses whether it be Main hand with Relic, or the Sub hand with Empyrians. This is not broken at all, and although I'd love my Spharai to 3x on both fists, and Vereth to ODD on both, it never will. This is intentionally coded in to balance it out with not allowing bonuses on offhand Ultimates.

[Taint2]

It's not just Empyrian and Mythic H2H weapons, Spharai is the same way, OTD is only for Main Hand. They changed ToM OAT H2H probably because you can dual wield OAT Magian weapons along with a kc, ridill, or other Occaionaly Attacks multiple times weapon. Now since H2H has always had a pseudo-dualwield effect but no OAT knuckles prior to this(we'll get to mythic in a second), they never worried about it. Every dual wielder can have weapon A with OAT and weapon B with OAT, so you have to allow both fist A and Fist B to have the OAT to make it fair.

Now why Mythic/Empyrian/Relic only proc on one hand you only have to look at balancing towards dual wield just like they did with ToM. In all cases of any of the 3 "Ultimate" Weapons when in the sub slot, or weapon B as we'll call it, none of these bonuses apply. You don't get 2x, or 3x Relic procs while off handing, you don't get Aftermath on offhand Empyrian or Mythics. H2H Ultimates only allow 1 hand to proc bonuses whether it be Main hand with Relic, or the Sub hand with Empyrians. This is not broken at all, and although I'd love my Spharai to 3x on both fists, and Vereth to ODD on both, it never will. This is intentionally coded in to balance it out with not allowing bonuses on offhand Ultimates.

Everything you said is correct but its still a huge disadvantage for H2H and dual wield weapons when compared to the 2handers.