"I wonder how many detainees are actually innocent and are not allowed to prove it."

They were involved in fighting US forces- they are illegal combatants [terrorists] or war criminals. I suppose some may claim [or refuse to admit] that they actually belong to miltary of some country or are for whatever reasons unable to convine the US miltary of their status- so they could be innocent in that sense. BUT if innocent in this sense, then this wouldn't mean they should be released- until their status can resolved they should not be released.
But if you mean innocent as in some civilian minding his own business and mistakenly identified as combatants, I find that very hard to imagine.
And as far as not being allowed to prove it, I doubt the US miltary is unwilling to listen to these guys- the detainees *problem* is they are the ones refusing to communicate- for whatever their reasons they may have. This is why I find it unlikely a innocent civilian could be stuck in there- unless he/she is simply extremely pigheaded or mentally incapable of speaking, there wouldn't be any reason why he/she would remain any longer than it took to confirm his/her story.