Even though the TP may be the Dark Ones "essence", that does not necessarily have to mean that he has full control of it. To use an admittedly crude analogy, my blood could be said to be a part of me, just as TP is a part of DO. But if I go to a hospital to give blood, I have no idea where that blood goes. Same could apply here.



Permission is required, but if Moridin&Rand are close enough, permission for one could be permission for the other basically. So the DO would be left with the choice of either taking it away from Moridin which would mess him up bad due to the addiction, or giving it to Rand.



Actually, I read a theory somewhere (I am afraid I cannot recall where but maybe the originator will read this) that pretty much suggested exactly that.

The idea is that only OP (being a naturally linkable power) activates the Rand/Moridin link. We have several viewpoints where Rand does suddenly get dizzy even though he was not channeling at the time. I can recall one with Min, and another where Cadsuane notices it and her ter'angreal does not register. The idea ofcourse being that at those times, it was Moridin who had used saidin. There is also that view where Rand channels and he gets a vision of Moridin who looks equally sick as Rand, further suggesting that OP use by either makes both sick. So why is Rand not sick constantly then? Because Moridin is using TP nearly exclusively now, which has caused the Saa to appear in his eyes very rapidly (Demandred mentions this). Why has Moridin abandoned OP? Maybe because it makes him so sick due to the link.

Does this necessarily mean that Rand could use TP via Moridin? Hard to say, but it does suggest that some kind of connection does exist that relies on OP. And even if TP use by Moridin does not fully activate the link, it is possible that Rand could still use it in an extreme situation. There is also the face(Moridins presumably) that flashed in Rands mind as he took hold of TP. The face-flash is similar to the link activation caused by OP.


EDIT: Found it. Luckers made the theory in Dragonmount. ts is a bit long but has strong evidence (with quotes) to suggest that Rand&Moridins OP connections ARE linked.
http://forums.dragonmount.com/index.php/topic,57372.0.html