I can't ignore the fact that Urethemiel's soul does seem to act as a key to one eluvian. This strengthens the argument that the old gods are connected to the Elvhen pantheon, but also makes you ask yourself - if Kieran is born (with DR) and Flemeth takes the old god soul from him then...
What does she do with it afterwards?
If the theory is true, then it probably means Flemeth knows about the old god souls being keys (to release any of the Elvhen gods)... If she releases the soul does it mean she wants the gods to be freed? If she doesn't, where is it? This also forces a branch in the storyline (if she doesn't release the soul) that is hard to bridge.
Lore tells Solas'Harel locked both god factions away, so if the old gods are indeed the keys to the release of somegods why would Solas create them (and how does one create a soul anyhow? He isn't a real god)? Doesn't it make more sense to him to use static keys that can't be manipulated? Or even destroy the keys or the eluvians? Does this mean he wanted to option to release the gods in case his plan backfires?
Besides that, if the old god souls are the keys then how come there aren't any Elvhen gods walking among us in DA already? Most of the old gods have been killed already. More likely I think he created an eluvian that unlocks with several keys, those keys being all of the god keys? More like what Sashimi said in the first part of the theory rather than the second part - like the doors in the Forbidden Oasis.
I can't ignore the logic in all of the arguments but I feel something's missing.
On the other hand -
If Solas goes against killing the old gods because they are the keys to the release of the gods - then it means he doesn't want to release them. Then what is his plan? Bringing down the veil?