Now I could see the argument for making it dex bonus instead of int for cunning feint, but pretty much everything else I think applying int is doing what it should.
The Swordsage could still qualify, it'd just be a later entry. I'd rather not have someone qualifying early with a stance that can be swapped out or picked up with a feat.Other nitpicks:
Requiring precision damage as a class feature precludes swordsages with Assassin's Stance from qualifying. If that's not your intention, you might want to change the wording.
The Within reach part of challenge is to allow for things that grant extra reach, such as becoming large. I don't think there's a lot of one handed reach weapons out there, but if there are any, yes it would function. Opportunist explicitly does not work if you attack with a weapon in your offhand, I would consider attacking with a weapon held in two hands as using the offhand, though I suppose I could clarify that.Opportunist should probably specify that the opponent must be in melee with the duelist to provoke AoEs. The way it's written, the duelist gets an AoO from across the room. And I guess the whole "within reach" part of the Challenge feature is written to allow spiked chain users, but it could use clearer wording.
Otherwise, great class!
There are several features that would work with a spiked chain and the like, but the majority of those (ie the control and damage features) are not features that a spiked chain user would actually want to use. It's a case of you can do it, but it's hardly optimal.