No, I'm suggesting that ITWF remain as is (granting secondary attacks on charge, AoO, and iteratives), but IMWF be changed to look more like it. So if you wanted to use a primary and two secondary weapons on a charge or AoO, you would first take ITWF and then take IMWF to get the additional secondary attack. Here, I'll just the borrow the format from Ziegander's post and write it up:
Improved Multi-Weapon Fighting
Prerequisites: Improved Two-Weapon Fighting, Dex 17
Benefit: Whenever you make a standard melee attack, a charge attack, or an attack of opportunity that would grant an off-hand attack, you also gain an additional attack with a different secondary weapon. This weapon may be a weapon wielded in a different arm (if you have more than 2), a different limb (like blade boots), or even worn (like armor spikes). If you do take this additional secondary weapon attack, each of your attacks suffers two-weapon fighting penalties. In addition the penalties to your attacks for fighting with two weapons are reduced by 2 (to a minimum of -0). Remember to add only half your Strength modifier to off-hand damage rolls.
Special: This feat may be taken multiple times. Each time you select this feat you gain 1 additional secondary attack. You may never use a secondary attack to attack with a weapon already used as part of the attack at your current attack bonus.
The writing there is rough and rather sucky, but hopefully it gets the point across. Instead of just taking IMWF and getting an extra weapon that you don't care about to act as your "primary" (so you can sacrifice it to use all of the attacks you do care about), you would have to keep taking extra feats to get extra attacks with your extra limbs. So a thri-kreen could take ITWF and wield two greatswords, or he could take ITWF and IMWF twice and wield a longsword and 3 shortswords. But the one with 4 swords would be able to hit with his primary and then have 3 chances to hit with a secondary to trigger one of the later TWF effect feats.
And while I was writing this up, Ziegander came in and posted...
Ziegander, were you intending to replace Multiattack with Improved Multiweapon Fighting? Cause I totally missed that, and I'm not sure that works at all because of the manufactured vs. natural attack progression distinction. Let me know if I need to write more on that, cause I know I'm sorta shortchanging it now, just out of time for a bit and want to hit post.