I was thinking about that as well, but wanted to separate the idea of manipulating dreams from seeing other's dreams. It is a fact that other Mandati can see what their brethren are dreaming. I think it is a small leap to concluding that any schoolman calling another should be able to at least watch the dreamer. The rest is pretty speculative.
We know, from Akka's POV, that he was at least able to pull his contact out of the dreaming state, and this seems like a form of dream manipulation. Extend that a bit farther, it might be possible for someone like Kellhus to manipulate one's dreams so subtly that the dreamer didn't notice, but this seems like a slightly larger leap of faith. But hey, it sounds like a fun rabbit hole to jump into. Lets take the plunge:
Akka's more unique but simple dreams, like Seswatha stubbing his toe over and over again for the whole night, could have been Kellhus testing this power of dream manipulation. He started with something simple, tested to see if it worked without Akka realizing he was being manipulated, then slowly transformed his dreams, taking greater and greater control.
Eventually, he made up the entire scene with Celmomas and Seswatha by the fire and inserted "Ishual" and laid the groundwork for the Map and The Coffers. Once Akka was convinced that was where he needed to go, Kellhus placed the Skin eaters at Morrow, and made sure Akka had just enough resources to get to the Ishual map but also so few that everyone else dies. Wouldn't anyone else knowing Kellhus' origins after all.
On a separate tangent:
As you mentioned, we know that Kellhus did something in Akka's head that allowed him to teach the gnosis to Kellhus. Lets assume that something was Kellhus interacting Seswatha. In this crackpot, then Kellhus could easily plot with Seswatha to transform Achamian's dreams, slowly at first but eventually drastically altering them, eventually leading Akka down the path his entire story arc has taken since that point forward.
Basically the same as above I guess, though with Seswatha as a co-conspirator.