Something about altering 'objective' reality with your subjective thoughts makes me feel like there is a paradox inherent in all magic that is likely expressed in some way through the utterals and innuterals.
It does seem unlikely that he didn't know that the skin-spy was there. Moenghus' goal was to 'unite' the three seas under one religion, this I am at least pretty sure of. What, then, was the goal of bringing the Mandate? I don't remember the timeline of events exactly, how long was it from the end of the way to when Maithanet and the Mandate showed up in Shimeh? If it was soon enough that Maithanet had already picked up the Mandate before the final outcome was determined, than its likely he didn't know Kellhus had won, which means bringing the Mandate was always a part of Moe's plan... It almost doesn't matter I guess. Either way, the Mandate's knowledge of the Consult would have been needed, and keeping them close at hand so their magics didn't get in the way would have been a must.
Still not really sure where I am going... So, regardless of who won, Fanim or Inrithi, the Consult would have been exposed, the Mandate brought to the side of the victor and raised up out of their centuries of irrelevance. Both Moe and Kell were/are Anasurimbor, and liekly would have went to Shimeh to meet the fellow. So even if the Mandate thought that Kell had won and showed up to find out another Anasurimbor was the victor, they wouldn't have been terribly upset.
Is it then reasonable to assume that the skin-spy might have just been an opportune coincidence? Maybe, it at least doesn't seem like a huge stretch. The whole point of revealing it was to do what?
Bend the knee of the Mandate to Maithanet and by extension the new Emperor? They would have done so either way.
Convince them to come to Shimeh? Again, they probably would have done that anyway.
Expose the Consullt? Already done by Kellhus.
Show them of their own fallibility? Probably, but to what end? They could have been convinced later of the awesome power of the new Emperor.
I can't really think of a reason for revealing the skin-spy at that moment. It just doesn't seem particularly relevant to me.