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!"

[Cilia]

snip

I specifically said "unsundered Ascians" and not Ancients for a reason - the memory crystals were made by the three unsundered Ascians (I don't remember anything about Emet-Selch being solely responsible, just that he made one for Azem despite their abdicating their seat in the wake of the Final Days made doing so taboo), but served different purposes between the Paragons and their sundered cohorts. To our knowledge none were used for that purpose during Amaurot's time, though Lahabrea sealing Hephaistos into one such crystal, Erichthonios making the one that instigated the Pandaemonium incident, and making one for Azem being taboo due to their abdication (which implies they were made for other Convocation seat holders at some point in the past) tells us the knowledge of how to make them was out there.

The Ancients' Final Days were slightly different than the one recently experienced on Etheirys. In the Ancients' time, the negative Dynamis forced their creation magicks to summon horrible monsters against their will as opposed to, well, whatever they wanted to make / summon (Emet-Selch specifically says that where prayers give rise to primals, the Final Days negative dynamis flood caused them to summon their deepest fears as Terminus Beasts / Blasphemies). Contemporary Etheirys' Final Days, due to the peoples' lower aetheric density, caused them to transform right out into Terminus Beasts / Blasphemies because they were more susceptible to dynamis-based attacks.

We know modern peoples' souls were also annihilated (or nearly so) upon transformation, but we don't know what happened to the Ancients killed by Terminus Beasts. Without anything stating to the contrary we have to assume that their souls were not annihilated and returned to the Lifestream. (The Terminus Beasts, specially powerful Blasphemies not withstanding, largely use the same model(s) as a cost saving measure. This is still an MMO, and reusing assets is far from atypical.) Besides which, as Meteion tells us in Ultima Thule, the souls of people turned into Blasphemies / Beasts (and those killed by them?) aren't annihilated per sé, just sent to her Nest so as to prevent them from being reborn. With her death (defeat?), presumably they will return to the cycle of souls.

The aether / dynamis relationship is more along the lines of matter / dark matter (or dark energy); I don't recall it ever being compared to antimatter.

... but yeah, don't think too hard about it. Magic, and they can always retcon or revise how it works to suit their story needs / interests.