the only two male forsaken left are him and Moridin. It does make more sense that it be Moridin though. He would have the authority to commandeer 13 channelers and 13 fades, though the fades aren't that big a deal. What really points to him though is the presence of that other dreamspike, he had two and Greandhal fooled with the other.

But he could just as easily have given it to Demandred.

Whichever it is he's there because the BT is going to be a major confrontation between channelers and if the dark can destroy as many as they can, and turn as many as they can, all the better.

Taim is taken orders from whichever it is, he's not there to take power just to wreak havoc.

I lean towards Moridin now though, more his style, as you point out it's not quite demandred's.