Interesting, though I still think at least initially that the word came from Dúnedain.
This was my thought as well. Kellhus is a terrible twisted version of Aragorn, in a way.
Solitary God = Absolute.
The Dûnyain have been inadvertently worshiping the Fanim God all along.
Wouldn't it be more correct to say the Fanim have been worshiping the Dunyain conception of the Absolute inadvertently? The Dunyain are older than the Fanim.
And Moenghus, yes.
Cnaiur's mother must have been a truly rare woman. Moenghus fathered a seemingly healthy son by her, though he was long gone by the time it was born, and likely didn't even know she was pregnant. Of course then Cnaiur's mother and the child were killed for being contaminated by Moenghus.