Of course they haven't been shown to be killable after the point where they were rendered unkillable, which is why I suggest the way to kill them is to remove the mechanism that makes them unkillable (since them being unkillable only applies to this post-stabilization context). There were things like them before the advent of time, and those things died. The only difference between the Daedra and those things that died is that the former is what we call them when the universe reached the point that they were no longer dying. Reverse that state and the daedra will become what they were before, ideas ebbing and flowing and fading away, dying.
But is that possible? Can the stabilization be reversed? I think that's up in the air. The Time Dragon has never really been killed (he was kinda-sorta killed when the mundus was formed, but were you really killed if you're still alive?), so we can't know for sure if it's possible to kill him.
Perhaps we'd best do with another source:
"These ideas ebbed and flowed and faded away and this is how it should have been. One idea, however, became jealous and did not want to die; like the stasis, he wanted to last.--Sithis
"These ideas ebbed and flowed and faded away and this is how it should have been. One idea, however, became jealous and did not want to die; like the stasis, he wanted to last.--Sithis
- "For ages the etada grew and shaped and destroyed each other and destroyed each other’s creations.... Here were the etada with their magic and their voids and everything in between and he yearned for the return to flux but at the same time he could not bear to lose his identity."--Vehk's Teachings
To me, this seems much more straight forward than you appear to think. According to Vehk, Et'Ada could die after Time but before Mundus. It seems that all you'd need to do to kill a Daedra is un-make the mundus. Destroying eachother seems like a straight-forward synonym for death, not a reference to destroying just an identity (like what happens in the dreamsleeve)
(and really, that makes one wonder, why would the creation of Mundus cause those not involved in it's creation to be no longer destructible?)
On a random note, the term "et'ada" seems rather poorly defined, and there is conflict even within unified texts as to what it refers to. Vehk's Teachings and the intro to the Monomyth posit all spirits before the creation of Mundus as Et'ada; however, the "Myth of Aurbis" and the "Altmeri Heart of the World" attribute the term only to those who created the world:
"Each gave birth to their souls, Auriel and Sithis, and these souls regarded the Aurbis each in their own part, and from this came the etada, the original patterns."--Vehk's Teachings
Similarly in the 'Altmeri Heart of the World', Auriel is created by Anuiel, and after Auriel's creation the etada begin to spring forth. Auriel is unique amongst the spirits (as it Lorkhan arguably).
- -"The magical beings, then, having died, became the et'Ada... The Daedra were created at this time also..."
-"So they created the Mundus, where their own aspects might live, and became the et'ada."--The Monomyth
"Each gave birth to their souls, Auriel and Sithis, and these souls regarded the Aurbis each in their own part, and from this came the etada, the original patterns."--Vehk's Teachings
Similarly in the 'Altmeri Heart of the World', Auriel is created by Anuiel, and after Auriel's creation the etada begin to spring forth. Auriel is unique amongst the spirits (as it Lorkhan arguably).