So here's my 2 cents on this matter, which is something I've been suspecting for a time now. I think the Elder One is the main antagonist of DAI, since he's mentioned in relation to ending of the world, and in relation to the Fade Rifts.Also:(May contain spoilers for Masked Empire.)
Also, the Forgotten Ones are the "evil" gods of the elves. From the Wiki. They were the ones tricked by Fen'Harel. The Forgotten Ones are early similar to the Forbidden Ones. There is four of them Anaris, Geldauran, Daern'thal and Fen'Harel. There are also four Forbidden Ones: Xebenkeck, Imshael, Gaxkang and the Formless One. Since Mike Laidlaw stated that the lore of the world is ambiguous and it's written from the perspective of the characters, and the actual lore is known only to people at Bioware, it is possible that these groups are the same, ancient powerful demons, only with different names to different people. Forgotten to the Dalish and Forbidden to Tevinter.
Anyway, long story short, I think the Elder One is the same as the Formless One, or otherwise known as Fen'harel, maybe.
I doubt it. Corypheus is a darkspawn magister, he was taught blood magic by someone else, either Dumat or the Forbidden Ones. He was a victim of the incident at the Black City, and not the mastermind behind it. Flemeth by all indication is more of an observer than an architect of events. So no, they are unlikely to be the Elder Ones.