On tempering I'd recommend you give this a read. It's ultimately a red herring, AFAIC, and mostly inserted to explain the Ascians 12k years later, because they still need to explain why e.g. Lahabrea and Nabriales acted the way they did, back before they had the SHB plot in mind. But with the primal 1) not summoned via the flawed rites the Ascians taught the beast tribes and 2) lacking a will outside of its controller, beyond perhaps self-defence instincts, it's hard to see what role it'd play, and it's given zero story significance at the time. The conflict divides the Convocation as well (touched upon here if you go to the French localisation of the SoS scene), which pushes against the hivemind interpretation of the issue many have, so provided that they acted as Emet did, and preserved their memories and identities, it may well have never become an issue for them.
Regarding whether Zodiark would succeed at restoring the star, he did succeed at that, as the second stage was a fait accompli, and succeeded in renewing the cycle of life. It's the third one which was not completed, but in all honesty, I see no reason why it would not, as the souls were preserved inside him.
The issue regarding the sundering is she 1) believed they would eventually go on a path similar to the Plenty and 2) were inherently unable to easily manipulate dynamis. 2) is debatable, because the story itself introduces a number of possible workarounds, including no less sundering - just not the entire star or race of beings, but on a more selective basis. There are other possibilities, like deploying familiars or other arcane creations without the design flaws of Meteion, exploring how the suppression field in Ktisis Hyperboreia could enable them to hone dynamis manipulation, etc. Zodiark's shield persevered for a minimum of 12k years, so it would've bought them plenty of time to experiment. 1) is also debatable, because they were not presented with concrete evidence of what was animating her concerns - she constrained what she revealed. I agree with this poster's interpretation of why that belief might've come somewhat a little too easily to her.
Agreed - and I believe they may have re-considered that stage until later if presented with good reason to do so - i.e. the evidence about the Plenty and Meteion.