it seems that you forget Avernus he already find a way to live for 400 hundred years as grey wardens, there is no urgent need to find a cure, since Avernus obtain a better result than simply find a cure.
Avernus is one of the reasons the HoF thinks a cure is possible at all, the other reason being Fiona, who actually was cured, through means even she doesn't understand or can replicate.
Also, extending one's life by sacrificing the lives of a half dozen other Wardens with horrific experimentation is less a cure and more of a...
...actually I'm not really sure what to call that aside from "sick."