My theory is that the elven gods are simply elves who, back in the days when there was no Veil, invited a spirit to fuse with them. Cole says in a banter that Solas is both spirit and mortal, and even questions why he would want a woman, to which Solas replies that once a spirit comes into the world they take on the feelings of the body they're in and asks Cole if he has not felt the same since he came down from the Fade.
Since DAO we've seen this spirit/mortal melding with mixed results -- Wynne, Uldred, and in DA2 there is Anders/Justice/Vengeance. Are the ones gone bad because of the Veil, or because of the mortal host? Even so, a good meld was probably rare, and that's why there are so few Creators. I think the Forgotten Ones are the melds gone bad -- they turned into beings of, according to the codex, disease, terror, spite, and malevolence.
Solas' views on Elvhenan are incredibly scathing (romanced Vallaslin dialogue among others), so perhaps these magister-spirit-gods even forced other elves to do it, and the bad melds were cast out, "Forgotten". But remember, Fen'Harel is the god of rebellion, as we find out at the Well, so we are seeing it through his emotions. Perhaps the young elf who took in the spirit of rebellion was even named Solas at one time. Perhaps he was originally the spirit of contentment and he just went bad the way Anders/Justice/Vengeance and Mythal/Justice/Vengeance did.
I believe that Sandal is the elven god June, the god of crafts, and when the Veil was created, perhaps by Fen'Harel, perhaps by someone else, he lost all connection to the Fade and became Tranquil. He mentions that he hears an old woman's voice, and most of us associate that with Flemeth. Why wouldn't Mythal try to contact June? Sandal is probably even an elven name.
Somebody is trying to bring down the Veil so that the Creators can return. Yes, Corypheus was trying to do that, but I think it was an accident and not what Sandal was talking about. What we don't know is if Fen'Harel is trying to encourage the Veil coming down, stop it, or find a different solution to the trapped Creator problem. I would lean toward Fen'Harel as trying to leave the Veil intact, but that's probably just personal bias. 