Guardian army only works on outgoing attacks, not incoming.
If the spell is supposed to guard on defence, then the spell should be fixed for that. If the spell is supposed to guard on offense then no, this idea would not make the spell void.
As for the guard option, I feel it should only be based as a spell and not be dependent on stats. I can see too many different types of abuses coming from this; an example would be a player could become immune and still gain from it. Guardian army allows the target to attack with a more power, but also stops the target from gaining the rewards. It is a pro/con balancing scenario.
I also think that it could be abused in fighting competitions such as heads (if it were ever to reappear), TC or other such competitions.
In short: I think the idea is alright, but it should be done as a WP spell -- or have guardian army clarified as to a defensive or offensive spell and then work from there.
Edit: Didn't see your post added as I was replying.