Yes, the interview established that the primal can control the degree of tempering - that is, where it's consciously desiring to temper. In Zodiark's case, there are multiple factors pushing against the notion that the tempering actively controlled their wills, the biggest being as you mention, that he lacked such a will. The lopporit suggest it is the power going into him that can cause tempering; and when Emet-Selch discusses it, he frames it in terms of alignment to the primal's dark aether and an urge to spread that aspect. Moreover, he was not created with the simplified rites the Ascians passed down to the beast tribes, which is what causes those summoners to want to spread their zeal. We learnt with Tiamat that even with such rites, the nature of the primal (Bahamut being her consort) meant it wasn't particularly pronounced in her case, although as it was removed her memories flooded back. Over time, tempering appears capable of causing memory/identity loss. So I believe that's why Lahabrea was hit the hardest after 12k years - because he made zero effort to preserve his identity. Elidibus still had his duty to the star and his people, from being a member of the Convocation, and that drove him. His refusal to replenish his memories is to avoid interference with his duty, so I believe in his case it's strength of identity to some degree that helped preserve autonomy. Emet-Selch worked hard to preserve both.
In light of all this, I struggle to see tempering as being particularly relevant, especially to the pre-Sundering Convocation. I suspect they just added it to explain the fanatical behaviour of the likes of Nabriales and Lahabrea. We understand that back then, they were driven by strong desire to want to reunite with those inside Zodiark, and that the ancients prized being able to return to the star, which such a limbo state potentially causing some distress to the souls inside him. Elidibus then emerges from him to mediate (and in the FR version of the SoS dialogue it is spelt out clearly that he does so because the Convocation itself was divided), and we can theorise that he could give voice to the concerns of those souls and prove that the withdrawal is technically feasible. I'm not sure what the invocation of tempering by some adds to any of this, because Zodiark is losing those souls inside him so this act is not strictly beneficial to him - and again, he has no typical primal will and certainly not the beast tribe primal type. The sacrifices of the "new life" are to replenish his aether reserves for the loss of aether he'd suffer, not to add to those reserves.
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here.