This isn't about a defense trapping. This is about being equipped to "redirect." With Riposte, the character is equipped with another weapon to deflect and attack. With magic, you need magic ready to go to redirect. For evocation, that is a Block.
Riposte is a mortal stunt. This is a full-fledged supernatural power. Furthermore, the character isn't using mortal magic. I suppose I could write it up as a form of sponsored magic, but the character doesn't create blasts of energy, do rituals, etc...which means at best it would be a very limited and specialized version of Channeling.
Balancing it by making it expensive isn't really balancing it. It makes it less available, not less powerful.
A wizard can't take a -3 redirect power in even a Submerged game.
Tell that to whoever wrote up Physical Immunity as a power that
can be taken in a Submerged game (with a +0 Catch, no less). Most of the powers in the game are designed around making them less available (through the limited resource of Refresh), not less powerful. The mechanics are designed to make more powerful abilities cost more as a balancing factor. Sure, a GM can impose additional restrictions...and many do. But the core system doesn't.
Also, I could see making a redirect spell using Evocation, though it would prevent the use of the deflection portion on successive exchanges. Create the Block with multiple shifts for duration. You block with it in one exchange, then use the rules on page 260 YS to turn the block into an attack (special effect is that you attack with what you were attacked with) on the following exchange. Evocation is a -3 refresh power - every wizard has access to a redirect power. It would cause stress each time the Block is renewed...but Evocation also can do a lot more than Block or Attack. That flexibility comes at the price of greater effort.
I haven't finalized the power. Three other posters have priced the ability at either -2 or -3, and I've tentatively priced it at -3...but I'm certainly open to increasing the cost.
And your argument against stress could also be used to say "evocation should never inflict mental stress," which is ridiculous: stress balances evocation/channeling magic.
Evocation produces something from nothing, can be tailored to bypass Catches, and has no hard limit to how powerful it can be. That is why it costs mental stress to use.
Mortal weaponry requires having the weapon in order to make the attack, cannot be tailored to bypass Catches on the fly (though you can bypass some with preparation), and has hard limits on the power level (you can usually carry around a Weapon:2, or a Weapon:3 if you're not concerned about concealing the item - Weapon:4 is very hard to come by).
The proposed ability (Karmic Redirection?) requires being attacked in order to make an attack in return on your next action, cannot be tailored to bypass Catches (because you're limited to what is thrown at you), and has a hard limit on the power level (about two points higher than what a pure mortal can have).
The fact is, allowing not just blocking but redirection with no preparation is broken. Allowing redirection without stress is even more broken. It basically negates enemy spellcasters and when the GM starts giving them this power, negates player spellcasters and in general makes them all pretty pointless. I guess "dog pile" tactics can still work, but that is really messed up.
But I was talking about the specific case of blocking an incoming spell. Your proposal talks about successfully defending against ANYTHING. And being able to send off an attack with no stress just by being attacked yourself (and successfully defending) is just broken.
There are two parts to the power, and you seem to be confusing one part with the other.
The first part is substituting Conviction for Athletics to dodge. "Successfully defending against ANYTHING" is a very misleading statement. Anything this ability can avoid with a Conviction of Superb can be avoided with an Athletics of Superb...all it does is swap one skill for another. That's not broken. By game mechanics, it's identical to what a pure mortal can do...make a defense roll against an incoming attack. It doesn't negate enemy spellcasters any more than taking Athletics (+5) negates those casters.
Second, redirection isn't giving you a free action. The action the character spends redirecting an attack could just as easily be used to pull out a rifle and shoot the attacker. Your objections seem to be centered around the amount of force the character can redirect, and the fact that the character can redirect magic. The former can be easily adjusted...heck, I could make it a Weapon:0 attack by default, which means the only damage would come from the shifts the character gets on the attack versus defense roll. The latter...well, I don't really understand the latter. Other than bypassing catches, there isn't much difference between a Weapon:3 bolt of fire and a Weapon:3 bullet. Both are equally efficient at doing damage.