Regarding Uthenera: It is actually a presumption that the elves of old retreated to the beyond for their endless sleep, because that is what the dalish believe. The dalish state more than once that a lot of the old lore is lost. It is actually also quite possible that they retreated into the same realm the eluvians link to.
Again, a lot of the elven history of old is lost. Maybe the creators at the time were no longer worshipped by the numbers they were used to and looked to others for that purpose. And seeing as how Flemeth/Mythal is able to turn into a dragon at will, maybe that´s exactly what the creators did, revealing themselves as dragons to the humans and taking on new domains. Flemeth/Mythal always talks about betrayal and the more I think of it, the more I believe she is referring to the betrayal of her fellow creators.
Endless sleep in realm different than the Fade? Ummm, on what presumption? The Fade and Uthenera mesh together perfectly, Fade - Dreamers - Endless Sleep. Solas spent at least thousand years in Uthenera, he's a Dreamer and expert on the Fade. Falon'Din was guiding the "dead" through the Fade, except ancient elves didn't die, they were Dreamers who went into Uthenera and there's much more of it. If everything is just misinterpretation we may as well assume that everything we know is false.
Creators trying to extend their cults crossed my mind, after all they waged wars between each other for believers [Falon'Din's case]. But no matter how much i try, i cannot wrap my head around it.
-If Creators were desperate enough to fight each other for followers, it would be logical to keep their intentions of converting humans in secret from each other, right? That explains not using their elven names or domains. Problem here is that cult of the Old Gods started relatively at the same time, so secrecy is pointless.
-First one to contact humans was Dumat, it's pure speculation but Silentir links Dumat with Mythal. Ok, number is not a big deal, Dirthamen and Falon'Din could be one entity, if we assume one refers to physical body and other to his Fade-ego. Bigger problem is the fact that Mythal was murdered, which i assume is the event that pushed Fen'Harel to imprisoning other gods.
-As i mentioned, Old Gods were still able to communicate with humans but not with elves, given how devoted were ancient elves (Abelas for example) i find it hard to believe they chose humans to do their bidding, beside how did they do it despite being dead/imprisoned?