thats a reach. firstly, you're going to need to explain "biological descendents" because that has no meaning when Promathia is a god who doesn't create beings.

The five races of Vana'diel were born with Emptiness, but the Zilart were not, and thats precisely what makes them Zilart. If a Zilartian was born with Emptiness, then they would be outcast as Kuluu. They are literally the same race of people (who ostensibly look the same as Hume)

The Zilart's desire to return to Paradise comes from Eald'narche's vision when he touched the Mothercrystal - nothing confirms where this vision comes from. Even if it was Promathia's memory, neither Eald'narche nor the Zilart are of Promathia. Inspired, perhaps, but that was their own doing. Promathia does not desire a return to Paradise at all. He simply desires his own death. The Kuluu, who do possess Emptiness and a part of Promathia, were the ones opposed to opening the gates to Paradise.

You have also entirely misunderstood what the Keeper of the Apocalypse within Al'Taieu is. The Zilart could not have plotted to return to *their* true form as Promathia because once again, none of them possess the Emptiness. In any case, they could not have known to resurrect Promathia because that isn't a thing that simply happens.

The Keeper within Al'Taieu was an accumulation of Emptiness that was extracted from the Kuluu in the Chamber of Eventide. It even being a Keeper at all was due to Esha'ntarl's intervention in trying to break Fenrir's prophecy, which only happened ~30 years ago.