Off-Hand Weapon Training can add more than 2 to an attack's stress if the character is dual-wielding weapons rated above 4. This can easily happen with enchanted items, and it isn't terribly improbable with Items Of Power either.
The Off-Hand Weapon Training thing is pretty resource-intensive. But if one is making a Crafter, and one has access to weapon 10 enchanted swords, then Off-Hand Weapon Training starts looking pretty damn attractive. Too attractive, actually. It's just better than other stress-boosting stunts in that situation.
If someone wants to dual wield IoP weapons, I would probably just make the weapons a set anyway, giving them a weapon rating that would represent (and possibly include) this stunt or something similar. Maybe at weapon 4 or 5 when wielding both of them, while each on their own would be a weapon:1. That way, the character concept is concentrated on 1 IoP rather than 2 or more, while still keeping the style of the dual wield.
If a player absolutely wants to dual wield swords of the cross, he better have a damn good reason for it. Plus, it would cost him 9 refresh alone to do so (3 for the first sword, 5 for the second, 1 for the stunt), which would mean, he would not be able to pick up any of the other true believer powers in a game that starts at 10 refresh, which kind of makes the concept itself broken. And even then, a sword of the cross is written as a weapon:2, if they are one handed. On the other hand, having a second sword of the cross would not give him any new powers, so the second sword can mechanically be seen as a mundane sword, there is no difference if he has 1 or 2. He would not have to pay refresh on the weapon, but it would only grant him +1 weapon rating for the stunt, which is good, but not overpowered.
I would probably try to keep IoPs in the lower weapon ratings anyway. A sword is a sword is a weapon:2 (or 3 as a 2 handed weapon). If you want to add to this, there are things like "True aim", which is on the swords of the cross. Or give the weapon a damage bonus like the strength powers provide, but don't make them flat out higher weapon ratings. That way, the stunt still makes sense and will always simply give a +1 to the weapon rating of an attack (because 1 handed weapons are thought to be weapon:1 or 2 at max), except when you can attack with 2 twohanded weapons (probably reserved for hulking size and/or strength powers), where it is a +2 and that is it. Of course that would still become problematic, if you are carrying 2 IoP with different special effects, but that would become a refresh problem again, very quickly.
In the case of the dual wield IoP described above, I would probably make them both a weapon:1, resulting in a weapon:2 with the incorporated stunt, and then give them a power that would boost their damage. Maybe a stunt that would add an aspect on a successful hit, that has to be tagged on the next attack and can't be given to another player. It would make the weapons sort of lock onto a target. Pretty powerful, once you have connected (or maybe inflicted at least 1 stress, "thirst for blood" sounds good for an IoP power) ,but you would still have to hit without the bonus.
If you have enchanted weapons, activation the enchantment is a full action, and I would not allow a double trigger on enchanted items. Even when using the stunt, that would be a weapon:10 attack plus a weapon:2 / 2 resulting in a weapon:11 attack. Yes, that is a lot, but 1 weapon rating really does not make much of a difference. If you would have 2 weapon:10 attacks on the same exchange, resulting in a weapon:15 attack, I agree, that is too much. Though I would probably not even allow the use of the stunt AND the enchanted item. The action is either about your super sword fighting skills, or it is about the enchanted sword, not both.
Swing For The Fences loses its drawback if the character is using an unconventional skill or an enchanted item to defend himself.
This one is simple to solve. Sure it says "weapons, fists or athletics", but I would simply have it say "on your defence roll", whatever skill is used. The idea behind the stunt is, that you can make a more powerful attack by opening your defence and making yourself vulnerable, and you are not less vulnerable from that move, just because you use a different skill.
Deceit or Intimidation stunts giving +2 to the creation of emotions become too strong if used with Incite Emotion. Stunts boosting physical attacks are limited to a +1 accuracy bonus, and there's no good reason why mental attacks should be easier to boost.
Your point about attacks is legitimate, but unfortunately it is directly contradicted by one of the sample stunts. Specifically, Infuriate from Intimidation. My current reading of this contradiction was that they meant to specify physical attacks. But it could be that Infuriate is just badly written.
There are a lot of stunts that give you a +2 when creating maneuvers with a skill. Suddenly this stunt is the equivalent of a social or mental gun, that seems very wrong. I would change it to only apply when trying to put a temporary aspect on the target. It would still be strong, and it could be tagged on the subsequent incite emotion roll, but it would take 2 exchanges to do so, which would be in line with any comparable stunt.
I think the most important part is to look at the spirit of a stunt, not so much at the wording (which is a bit off at times). I know, players and GMs discussing this can be like a fay bargaining, but I think any of the solutions above leaves the stunts viable, while removing the part that would be overpowered or badly worded.
I hope that helped. If something is unclear, please ask.
