I believe that the taint is connected to the seven seals. My arguments: Only saidin is affected and not saidar, why? The taint came into effect just after the bore was sealed, with saidin only. If the DO could have tainted the source all the time, why didn't he do it before? Some prophecies say the dragon will break the seals (i don't have the reference with me), why should he do that, if not to clean the taint. LTT wants Rand to break the seals (sorry don't have that reference either), is it only madness...or has he perhaps realized the seals connect saidin in some way to the DO and make the taint possible. As to the last battle: In the strike at shayol ghul story we are told that the original plan called for male and female channelers, perhaps that would have prevented the taint and perhaps that will happen at the last battle.