*Enter horse bludgeoner*
Just a few thoughts here. One is that two weapon fighting also gives you double the opportunity to score a critical, or if you carry magical weapons, allows for multiple effects. I think an accuracy trade-off is especially acceptable then if you assume only highly trained (i.e. high combat bonus) beings can use the approach effectively, and makes having two attacks meaningful.
That said... as to thinking the two-weapon method used is a bad one in regards to True20's trimmed down approach, I can see your point. I don't mind it at all, but I can see how one could. If I had to change it, I would probably make the Two-Weapon Fighting feat work like this:
Make it widen Crit range of primary weapon by one if off-handing a light weapon, and by 2 if offhanding a medium weapon, but make the auto-miss (or fumble) range an equal value to crit range. It is easier to do something stupid when holding a couple weapons

Allow a +1 to parry as well and do away with the 2 Weap Defense feat like you planned to. Defininitely not stackable with Improved Critical, though. That would be madness. I would also probably use fumble rules here too, like making increased auto-misses negate ALL Parry bonus until your next turn in the initiative chain, or worse hehe.
So it's like a Parry +1 feat that makes it easier to do more or NO damage. Just a possibility, though I make it sound more convoluted than it actually is.
