(Spoilers)The rejoining kinda useless.

[Cilia]

I believe the idea was that Zodiark, the god that can rewrite the laws of reality, would fully restore the dead.

Yes, but Through His Eyes states that one of the few things the Ancients were unable to do is endow their creations with souls.

Zodiark may have been created to be "the will of the star," capable of rewriting the laws of reality... but that's what his creators wanted him to be, and as we know primals aren't the real deal (i.e. Shiva is not a recreation of Saint Shiva as Ysayle intended; she's just what Ysayle believed Saint Shiva was / is like). The Ascians call Zodiark "the will of the star," but whether he is in sooth is... arguable; and if the Ancients couldn't endow their creations with souls, why would a creation of theirs be able to?

I think the Echo has a limit to what it can protect against, in terms of Tempering. Most primal fights are over in a manner of minutes for us in real time, which may be hours for ingame time (depending on whether or not they consider time flowing during trials to be normal time or not). No primal has lasted longer than that, and no Elder Primal has tried to influence us over a longer period of time. Rather than loosely defined, the Echo has merely not been pushed enough when it comes to being tempered.

Zenos conjectures that the Echo doesn't immunize people to tempering, but that it strengthens one's ego so as to withstand a contest of wills with primals.

This is somewhat supported by new information from Shadowbringers; talking to Emet-Selch at one point has him admit that he and the other Ascians are tempered by Zodiark and were when they summoned him because his power was simply too immense. If that's the case it would simply take a powerful enough primal; the ones we see summoned are simply too weak to overcome the Echo. (Shiva is able to overcome Ryne's will when she channels her in E8, though Ryne is able to avoid being tempered and just ends up as an out of control primal.)