Major Spoilers for all Expansions - Some general things I didn't understand

[Alleluia]

I'll cover 1 down below.

2. This is just conjecture because I don't think it was ever touched on heavily by the story, but the gist of what we've been given is that every living thing has a natural expiration date (even the Ancients), and living beyond that will start to cause memory and personality degradation. This is why the unsundered Ascians needed their memory crystals; unlike their sundered pawns who use them to remember their Ancient selves, the unsundered Ascians use them to keep their memories and personality straight because their unnaturally long lives will otherwise take a toll on them that reduces their cognitive abilities. By the end only Emet-Selch is still using his, with Lahabrea having seemingly given himself over to his Hephaistos "self" and Elidibus having abandoned it to focus on his duties; and Emet is by far the most effective for it.

So to directly answer the question: theoretically but there's still a limit to how much memory individuals can hold, so a longer life won't add to the Lifestream.

2a. To my knowledge the plan had nothing to do with memories, everything to do with nurturing the planet to use some amount of its living things as blood sacrifice to restore the Ancients who sacrificed themselves. The Final Days didn't destroy the Ancients' souls as it did with the victims in our own time, and the planet was in an inhospitable (if not uninhabitable) state but it still had all of its aether. Presumably. They just had to nurture the planet to produce enough souls to sacrifice to Zodiark for what they really wanted.

Did Zodiark respect thermodynamics, and thus require more aether to produce whatever you wanted since entropy is a thing? Well that I couldn't tell you because it's magic, and magic works however the writer(s) want it to work.

Can you direct me to the source that says Emet was using his crystal and that the crystals were actually given to the amaurotine officers pre-sundering? I was under the impression that crystals were something the unsundered made (with Emet doing the actual crafting) to facilitate the raising of the sundered soul shards to their seats and jumpstart the recovery of their memories. Not a pre-sundering thing the convocation regularly did. Otherwise, there wouldn't be just the Unsundered's memories of Hermes in the Fandaniel crystal, for instance, but also his predecessor's memories. Am I forgetting something in Elpis or Amaurot that touched on this?

And re: the final days -- the ancients were just stabbed/chewed to death etc, yeah. But the creations that their creation magicks made were blasphemies. It seems likely to me that that caused at least some loss of aether. The aether is used to manifest a creation which is then immediately annihilated by dynamis and turned into a blasphemy? Or perhaps the blasphemies are just a result of the dynamis reacting with a part of amaurotine's aether, but it couldn't turn the whole soul b/c of their aether density being too thick? Either way, the "aether to no aether" thing still happened. Or else the blasphemies wouldn't still look the same. Its like the dynamis attack was only able to peel off a layer of aether from them about the density of a familiar and then corrupt that. And then in modern day we all have the density of an amaurotine familiar, so it just corrupted modern people whole.

I'm still not certain whether I buy that the aether is just annihilated when confronted with dynamis, cus in an antimatter/matter annihilation reaction(which this has been oft compared to) BOTH things are annihilated. But as we saw: there was a big old monster, animated by the emotions of the person it used to be, left behind when the dynamis attack hits. So something must physically be there, and (barring some unknown third force) that something must be made of dynamis if there is no aether present, thus dynamis was not annihilated in the reaction. So, I think it makes more sense that aether and dynamis can become one another and the aether just experienced a state transference (like matter to energy transference). But this is just me rambling now. >_>

Or as you said, the writers are looking at this going "Laws of thermodynamics? In MY magic system? Ha!"