...and back to Meppa.
There is an assumption by a lot of people that Moe is some sort of super user whose influence is still driving events in Earwa. I think this is unlikely.
Moe's actual power: We have two perspectives on Moe's power. 1) He is weak as a Chisaurim (from Kellhus). 2) He is the ultimate Chisaurim (From Xerius's meeting with Mallahet and this is assuming Moe=Mallahet, which seems most probable). To me, it seems most likely that if anyone were fooled, then it would be the world-born men of the Nansur and NOT an actual Dunyain. I suspect it's possible that Moe did wield considerable power, however, I think that power was political and not arcane in nature. Case in point: The Scarlet Spires are often referred to as the most powerful school in the Three Seas, yet we know that they are anagogic sorcerers and their arcane power is nothing in comparison to that of the Mandate. Therefore, this power is either political in nature or a reference to their number of sorcerers of rank. It logically leads that the Nansurium's fear of Mallahet/Moe is fear of his political power which is in turn gained from his ability to influence as a Dunyain. His very words may turn Xerius's heart. Beyond that, isn't it Skaeos that says Mallahet is dangerous? Skaeos who we all know is a skin spy. If it is Skaoes that says this, then it leads that his "kind" would be terrified of Moe anyhow. There can be little doubt that the Consult knows anyone that tries to follow him disappears.
The actual power of the Psukhe: We are told time and again in the series that the Psukhe is the least powerful of the sorceries. I know that such a thing as an unreliable narrator exists, but it is generally considered a cheap tactic in literature, and strikes me as something a writer of Bakker's caliber would try to avoid. I chalk up contradictions in the glossary to mistakes by Bakker or things that he just wanted to change rather than the result of unreliable information from narrators/Threes Seas historians. At the very least, the Psukhe is described as being the least refined or the sorceries, a wooden cudgel to the gnosis's steel sword. And, as I said above, Moe was weak in the water. For argument's sake, if he were strong in the water, I still don't believe the Psukhe is sophisticated enough to perform a soul transfer within the magic-nullifying circle of a chorea.
All that being said, Moe's power was too limited for him to still be directing events 20 years after his death. He chose the wrong path as a captive of the Scylvendi, and he wasn't able to deviate from that path. By the time he made it to the Utemot, he likely had no idea magic existed or that it was something he had to account for. He scarred his arms so that he could travel across Scylvendi lands. Realizing that this would make him unwelcome in the Nansur, he picked the only logical path and went south to the Fanim. There, he discovered magic. He had no choice as a Dunyain but to dominate his circumstances and learn magic. At this point he had no way of knowing that the Psukhe was the least powerful magic as he'd likely met no one to tell him otherwise. He blinds himself, believing it would make him more powerful in the Psukhe, and probably discounted the notion that it took passion to carry the most water (just as Bakker made us think that gods may or may not be real until we learn for sure they are in TJE). Blinding himself takes away some of his Dunyain ability (If it was an advantage, all Dunyain would be blind), so he is left with weak magic and weakened Dunyain powers. He is still able to orchestrate the events leading to the Holy War, but a lot of that is done through proxy (Maithanet).
I'm saying all this to point out that I don't think it is possible that Meppa=Moe (or is a vessel for Moe's soul) and while I think it is probable that Meppa is one of Moe's former disciples, for the current story I think it is unimportant. I don't think Moe is still directing events, either from beyond the grave or through the sheer intricacy of the plans he laid 20 years previous. Meppa is in the story as a foil for Psatma. I don't think his previous involvement with Moe has any bearing on what is happening in the AE. So the real question is not "Who is Meppa," but rather, "Can Meppa stop Yatwer?" He is, after all, the only one left to defend his faith. Fanayal seems more interested in plowing fields and building an empire.
My 3 cents.