Personally I'd consider that to be plastering on another badly written story aspect just for the sake of Lahabrea bad, which is well past boring at this point. Part of the issue here is all the things tempering can be. If we strip it down to its most basic, i.e. aligning the summoner to the primal's aether, Zodiark appears to have done that as a minimum, but by all appearances not much more beyond that. I suspect the reason they were tempered is because of the sheer amount of power that went into him. At the same time, its consequences appear to be limited to loss of identity and memory if the summoners don't make an effort to preserve these, as Emet did. But at the same time, he is not the type of primal with a will that compels their summoners to do anything, especially since Elidibus is in his driver seat and without him, he's more or less an automaton. For the sort of tempering we see with the beast tribes to manifest, it is not mere alignment to the primal's aether, but down to further elements: 1) flawed rites that instil in the summoners a desire to spread their zeal by tempering others and 2) the nature of the primal (=desire to compel servitude) and an associated desire to temper beyond the original summoners where the tempering is a mere side-effect of the summoning. Ramuh for example lacked aspect 2, but still exhibits 1, so his summoners still have that desire.
The point here is that there's many aspects to tempering, and not all forms of it are made equal, and none of it would specifically require a plot like the above. Zodiark's tempering primarily appears to be invoked to explain why Elidibus and Lahabrea suffered memory/identity decay over time, to explain dialogue from ARR and HW in all probability that needed to be re-purposed. Meanwhile, Emet-Selch is quite able to deviate from the original plan where he deems it necessary, because the plan was never formed out of servitude to Zodiark but in order to restore the broken star.
I don't think the explanation given with the Loporrit dialogue was the best, because it is clear that regular summonings also involve aetheric corruption and it's not just a matter of zeal, so we have to assume there's other aspects here as to why tempering is not an issue for these specific summonings (maybe the primals are not potent enough relative to summoners for it to matter, for whatever reason.) This is supported by what Rokke mentions, as in the Japanese version it is indeed Zodiark's power which the Loporrit mention would make the difference, so it'd suggest even if you use the correct rituals, if the entity is powerful enough it can affect the summoners' aether.
Having done them, I can say that there is little suggesting it one way or another regarding their own origins. His "test" is formulated out of little more than spite, IMO, and he is quite happy to damn his supposedly beloved creations with the rest of the universe when it suits him.Hermes has been argued to death that his actions don't match his words. The way he treated Metion does not match to what he preaches. It almost seems that Hermes himself doesn't even know what he's talking about. He's upset that his mentor would chose to retire, then changes his tune a bit saying "death is only beautiful for man". Metion can't eat, which shows he didn't intend for her to live her only life, only live for her purpose. His ending statement about how man judges all life, so he as the chief of Elpis, will judge man. Not all life was judged though, since it's presumed there was non created life that existed. Man can't be judged as a creation because they aren't a creation...or are they? I haven't done the side quests, but if the ancients could transform then they could easily alter anything they don't like about themselves. Could they have potentially over time, altered themselves and passed that on through generations? I'm wanting to sit down and do all the sidequests, so maybe this is already answered, but I find it strange be are presented more background information about the 5th zones races then we have with the ancients.
She implies during her dialogue that he was so much more potent that she had to do the latter in order to do the former. It fits rather poorly with the post-Elpis scene, because there you get the impression her ultimate goal was to sunder the ancients, one and all, either way, but I suppose in her original plan maybe she hoped to get away with not doing it to the star and being strong enough to defeat Zodiark without the need to do so... only he was much more powerful, even without a driver (the "wisdom" of her plan notwithstanding.) That cutscene is pretty awful because it glosses over key aspects of the conflict between her faction and the remaining ancients as originally detailed, and also makes her seem unreasonable in my eyes.Hydaelyn not only defeated Zodiark but completely sundered the world.
Anyway, all interesting questions, but I doubt we'll get answers to anything like a majority of those.
No, they had emotions, and it's explored in quite a bit of depth in the sidequests. It's more that it took stronger emotions on their part to influence the entelechies because of their dense aether. But the idea that they lacked emotions is not supported by the lore, nor is the idea that they had "super basic" interests. As for Hermes, he could not understand the concept of life having meaning in the face of death, which remains a facet of his persona as Amon, a sundered being, which Xande's nihilism upon encountering death reawakens in him. Little did he expect that no other civilisation would provide this answer...