A few thoughts

MSQ



If what was said by Venat was true, Elidibus was "gone" at the first summoning of Zodiark to become the heart. What we also know is that Venat seemed to be the only sacrifice from what we know if the other member who met with her asked "why her' and she would be missed? Venat/Hydaelyn later used Minfilia as her mouthpiece - the two merged?

So if Hydaelyn could do that, why not Elidibus? Essentially using another member who rose to the office as a mouthpiece?

The other possibility by looking at how primals are summoned since taught by the Ascians who summoned Zodiark is that there isn't a "heart" for Zodiark. I'm just not sure about that theory tho.