Definitive, no. And of course the theory could be flat wrong. But a lot fits:

1) the Old Gods appeared very shortly after the Elven Gods disappeared,

2) the Old Gods taught the Tevinters a form of magic that is an imperfect version of that of the old elves,

3) the Elven Gods appeared able to take on the form of dragons,

4) there were nine Elven Gods: subtract Mythal and Fen'Harel and you have only seven,

5) Solas irrationally despises the Grey Wardens, i.e. the only people who can permanently kill an Old God/archdemon,

6) Mythal is very interested in retrieving the soul of the archdemon that was in Kieran, and flat out tells Morrigan she doesn't know what she actually has

7)the Elven Gods could flit from body to body, so long as the new host was willing: the Old Gods/archdemons can do the same with Darkspawn, but die if they try it with a Grey Warden (i.e. an unwilling host)

Again, nothing definitive by any stretch, but I find the theory fun and interesting.

That's another possibility. I hope though that there simply never were any Forgotten Gods (they certainly seem to be incredibly vague: we literally know nothing about them at all, not even the smallest hints), but that they're just an invention of the Dalish to handwave away the darker sides of their infallible Elven Gods. We know at least one Elven God hunted people as well as animals, and that she was insane, even if the Dalish have forgotten the former and try to pin the blame for the latter on the Forgotten Gods. Another one slew all the creatures she created save one, something completely forgotten by the Dalish.  I can easily see a scenario where the Elven Gods turned on each other, and the Dalish invented a scapegoat out of whole cloth.

This makes me want to buy some novels if the franchise has any. 

Has a few, plus some comics. Not sure how much of the lore is explored though.

The Solas bit is funny too. He contridicts himself in the entire plot there. He's very pro-spirit/demon all the way through yet when the Grey Warden stuff happens he even says 'The demons are a means to an end.' He accepts that part of it taking control of demons but not their ultimate goal. He doesn't care about the needless sacrifice needed to do it. Just the end result, killing the Old Gods before they are even Archdemons.

Then again, I find it funny that the game seemed to be trying to hint to us, the players, that Grey Warden's are secretive or bad to some degree, yet all of us still remember DA:O where the Grey Warden's were the only ones trying to save the bloody world.

I don't think that's ever been a secret: even in Origins the Grey Wardens were portrayed as paranoid assholes who few liked (though many respected). That their secretiveness almost led to the end of Ferelden as we know it is obvious, and even the Orlaisan Grey Warden makes a point of saying that the Wardens are often cold and distant with one another.

Besides, the stakes in Origins were never as high as the entire world. The worst that could have happened was the devestation (not even destruction) of Ferelden: the rest of the world would have taken out the Blight sooner or later. In fact, seeing as how the Blights are getting progressively shorter, it probably would have been Sooner.

Wow, I definitely will need to play the game once more. Reason being, is that Solas I swapped out Solas for Dorian. I set up Dorian to be a mixture of spirit and fire spells and when the first dragon came along I ended up swapping him for Dorian so I could customize his skillset (I wasn't about to pay $1000 to strip him of his skills). Eventually I forgot about it since Dorian had such an interesting storyline. I also regret not bringing Solas to the sacred elven keep. He was rather sad about not being there when I spoke to him. Then again...he's always sad.

I took him along 2nd play through. It was okay at least. But apparently there is a glitch meaning when it's his chance to voice his opinion on the well thing, he doesn't but is angry with you later and how he begged you not to do it. This confused the hell out of me, because he didn't because of the glitch. Not sure if that's fixed yet.

I wonder if you can get Solas to drink from the pond since you can stop Morrigan from doing it if you wanted to. It might just change his storyline a bit. I remember seeing an option for you to use one of your support characters as a suggestion when you question morrigan about drinking from the water.

Nope, it's always you or Morrigan. Choo-choo!