If the armor could never use the stats of the person who is wearing it, why would the armor care about being worn at all? It gains no benefit from being worn, only downsides (it now has to compete with somebody else for movement).
Animated armor has stats because it is assumed to be animated (move on its own). Your version of animated armor sounds more like you're trying to create some sort of cursed armor which will try to take control of the person wearing it, in which case it doesn't truly need stats of its own, it's controlling somebody else so it enjoys the benefit of having the statistics of a scary hero.
As for the mechanics to implementing this control and how to damage the creature, it is most likely a lot more effective if you implement it as a mind-altering effect than a grapple. Not only is the kind of character who tends to wear armor going to be far more likely to fail at resisting a mind-altering effect than a grapple, but it also makes a lot more sense. A single grapple result would indicate the armor "overpowered" the warrior and keeps him grappled, but that doesn't suddenly allow it to control all its movement. Armor and fighter would be in constant struggle against eachother, making it very hard to fight other targets. They are -both- under the grappled effect, the armor has no real action except to continue to hold the grapple.