The Allagans didn't make any further progress on tempering because Amon/Fandaniel exiled Owen when he tried to work on it. In fact, Fandaniel notes that Nidhana's talismans are similar to an experiment that Owen himself conducted, during the Lv.80 quest 'Shadowed Footsteps'.

I think the main problem with aetheric corruption is that while you can restore aetheric balance, it's less simple to restore someone's sense of self after its already been lost. What makes the Void's denizens really interesting is that it seems like several of them retained their sense of self despite being aetherically corrupted. Perhaps that has something to do with Eara's observation about the soul always being 'umbrally charged' in traditional tempering, regardless of the causative element. Either way, we'll have to wait and see what explanations they come up with.