Off-hand weapons training is not going to compare favourably to any of those proposed stunts, either, except in the cases where it trounces them thoroughly (see above re: bad stunt design).
Stunts are supposed to be situational. Though I might be misunderstanding what you mean here.
That's your interpretation and your preference in your homebrew stunts, and that's fine. Off-hand weapons training will still compare erratically to more restrictive +2 stress stunts.
Erratically, yes, which means sometimes good, sometimes bad--it's a stunt that not everyone will want to take, which is fine. A stunt shouldn't necessarily be something that fits every character.
No, in the absence of a +2 stress stunt, you've got a character using a sub-par weapons choice benefiting from a stunt that partially compensates them going up against a character using a comparatively optimal weapons choice who has been assumed not to have any stunt at all. This is a horrible comparison.
If you want to make a fair comparison, include a stunt for the swordsman that you would deem reasonable (and that you have not selected with the apparent intent to skew the results as with the case of leaving out any stunt at all).
Fair enough. I was comparing it weapon-style to weapon-style. I'd say, all other stats being equal, maybe the broadsword user has a +2 to stress, at the cost of a -1 penalty to defense when he attacks. That would be a reasonable stress-adding stunt that gives him an advantage in power at the cost of something that potentially lets a knife-wielder hold his own (especially if you apply that penalty to defenses against maneuvers as well).
And a dual-wielding knife user might not have the damage output of even a stuntless broad sworder, but damage isn't all that makes something "optimal." The knife user is going to be able to get his knives into a lot more places than someone with a broad sword, for instance, which is the advantage of low-stress weapons.
That's a valid interpretation, and I might be inclined to agree with you. It does not, however, change the erratic nature of Off-hand weapons training's comparison to more conventional stunts.
Nowhere does it say a stunt has to benefit everyone equally. A stunt like Takes One To Know One compares unfavorably to a straight boost to Empathy if it's close in value to the Deceit skill.