I think there is a likelihood that everyone in Thedas is talking about the same thing but with a slightly different outlook on it. Sebastian even admits as much when talking with Merrill. She says they believe the elven gods will start talking to them again when they remember what it is to be elvish and of course the Chant of Light says the Maker will only return when everyone follows the Chant of Light. So Sebastian says that may be they are both talking about the same divine force.
I suspect the elven gods are actually benign fade spirits and the forgotten ones do correspond with the old gods. They are likely dragons that were possessed by demons to gain an entry into the material world but may have been "intelligent" dragons in the first place, which is why they attracted attention to themselves. Whatever the case, it would seem that they taught mortals the secrets of blood magic. I say mortals rather than simply humans because there are varying accounts of who they learnt it from, the old gods and the elves themselves. If you assume that the elves resorted to blood magic in response to the threat from the humans, this might account for why their "gods" appeared to abandon them around the same time as the benign fade spirits either no longer wanted to be associated with them or had actually been frightened off by their association with demons. In Asunder the lesser "good" spirits appear to flee in the presence of demons and it is only the powerful faith spirit that is confident enough to confront one. If the elves were demonstrating lack of faith in their own gods by resorting to blood magic, this would account for why no powerful benign faith spirits came to their aid.
The Tevinter definitely worshipped the old gods and it is said they used dragons among other things in their battles against the elves, but these were probably ordinary dragons that they controlled with blood magic. May be after the elves were defeated, the dragon gods went underground to hibernate and at this point were sealed in, possibly by the Maker/Fen'Harel as the tales suggest. However, they could still communicate via dreams with the Magisters and this is why they encouraged them to attack the Golden City, hoping it would free them. When the Magisters became corrupt, they could still hear the song of the old gods, as could their followers and this is why they continued to seek them out, partly out of a desire for revenge and partly because they couldn't help themselves. In Awakening the Mother goes mad because she can't bear no longer being able to hear the song. When the darkspawn reach the old gods, they do in fact release them, but in the process taint them. However, I think it is no coincidence that they are known as archdemons because that is what they were in the first place.
I'm pretty certain the old gods must have be present on the surface initially because otherwise how would the Tevinter know that they were being attacked by one of their own gods. It was the fact that Dumat, the chief of their old gods, was leading the darkspawn against them that caused such a crisis of faith.
Someone has suggested that Flemeth may actually be the Lady of the Skies that was revered by southern barbarian tribes. In one of the stories it talks of how she carried a group of them off to help them escape their enemies, which sounds an awful lot like Flemeth. Flemeth was a real women at one time but at some point became possessed/united with something else, probably much as Wynne was united with her faith spirit or Anders with Justice. It is, of course, entirely possible that what she united with was one of the old gods, hence her ability to shape shift into a dragon. It would also account for why she formed the plan, pursued by Morrigan, of trying to save the soul of the old god, and regarded the darkspawn as a threat to herself.