I'm not even sure they can die, in a way that humans would recognize. Syla wonders if the reason Apostles seven through twelve are so distant is because they don't exist anymore. But she says that after the MC has made contact with three of them. That says to me that Syla doesn't expect a god's "death" to actually end them. Given the eldritch horror themes around the Apostle, I think it's probable that the Apostles die the same way that Cthulhu dies. You know, "that is not dead which can eternal lie", where what looks like their death is just them retreating from the world to sleep and heal and eventually return. They get closer to or more distant from reality, but they never really go away.
Your thoughts are pretty similar as mine. I'm of the opinion that the Apostles are fundamental aspects of the universe. In the sense that when they lie dormant, they fall in sync with all of the universe. Which means you can't really kill them, as shown by the 7th having to do it over and over again.
Ella mentioned during her 5th Evolution that, presumably, the 3rd Apostle should go back to "sleep" like the others. Implying that they have periods of inactivity. Valravn, lamenting that the 7th hasn't been answering his call earlier could also be an example of this. Along with the need for Chosen to rule over monsters for them in their absence. Then I believe Tanos and Deryl talked about how traces of the Apostles can be found in everything in their universe. Meaning that they have some connection to the universe in a material way.
The reason 8th through 12th are distant could be because they're not sentient at the moment and rejoined back into the whole of creation for now. Possibly due to the 7th constantly killing their avatars over the years and it temporarily knocking their consciousness out for a while. Or it could be that their spheres are farther away or were moved away. Compared to the 1st - 6th. Authority, Power, Body, Memory, Time and Space spheres could be much more closer to the universe, especially since they were "born" first.
So it's a combination of possibly being farther away than the first six, either by design or because someone or something else caused it. Along with their brief bouts of slumber for unknown duration. Either way, I don't think they're dead or can truly die from anything below their cosmic position. Especially since the 7th can't even do it permanently. Which would leave only the Origins or Arbiters and the Outer Twins currently.
