The Grey Wardens want to find all the old gods and kill them before they are corrupted into arch demons and make another blight. Orzimar wants to reclaim their lost homelands. Qunari after the events of Origins are now, at least somewhat (if Sten was alive to report it), aware of the threat of a Blight and how close the last one was to taking over.
We know there are maps through the deep roads so I'm curious why they don't ask the Qunari if they could send some of their bomb specialists to place a crap ton over key areas of the deep roads and collapse it on top of the Dark Spawn from the safety of the surface.
I kmow the Dwarves want to retake it, but it's clear they are losing more ground then gaining and I imagine it would be easier to rebuild a deep roads section rather than having to battle it from dark spawn and clean all the taint. And if it's about preserving their ancesestors' work I'm pretty sure time and the Dark Spawn have already desicrated it.
Wardens I'm sure wouldn't mind taking out Dark Spawn this way. And if the Qunari are the ones setting the bombs I don't see why they would take much issue.