Kieran is cured dragon 
Kieran has the soul of an Old God, which originated from an archdemon. Archdemons are tainted Old Gods, and the only part of Kieran that is from the archdemon is, as I said, its soul. The taint corrupts to body and mind, not the soul. It's a physical corruption.
And Old Gods are dragons like Flemeth is a mage. Kieran is a very, very poor example, at best.
Fiona cured. Cory is human Sethius Amladaris. He not born darkspawn. And darkspawn, greay warden are all the same blighted creatures.
There is absolutely nothing to suggest a Darkspawn can't become a human. Oh wait. There is all awakening game. We just need to go further.
Cory is a darkspawn, not a ghoul. A ghoul is a human/elf/dwarf/qunari who has been tainted beyond recovery, while a darkspawn is a separate being. Grey Wardens are still tainted, it's just that their corruption progresses slower. The Wardens, despite the fact that they are the most knowledgeable group on the surface regarding darkspawn and the taint, know of no cure to the taint. iirc, Fiona being cured was never cleared up in terms of how it happened, though I could be wrong about that.
As for turning a darkspawn into a human, that's about a likely as changing a dwarf into a qunari. All we saw in Awakening was that the darkspawn could gain consciousness through a reverse version of the Joining. Despite possessing independent thought, they are still darkspawn. There's no 'cure' for being a darkspawn since being a darkspawn isn't a disease.
Regarding Avernus; iirc, he studied both advanced blood magic and the effects of the darkspawn taint within Wardens, using a combination of the two to create his concoction and extend his own lifespan for several hundred years.
I do agree that the taint and the darkspawn should be explored more in depth at some point. It's a section of the lore that's been ignored for the most part.