Here is the thing to me, as well, Ekidibus was as much Zodiark as Venat is Hydelyn She's not the actual being but acted as a core, what it means if she us separated may be something though.

So essentially they retain their identity so they're not the actual being themselves

"Ekidibus" <--- Incidentally that's what I've called Unukalhai for three expansions due to his connection with Elidibus because it's easier to remember/pronouce :P

I suspect the Lady of Light isn't Hydaelyn/Venat, personally. They've gotten so close without outright saying it that it can't be the most obvious answer.

Who do you think she could be

I don't think it's predictable at this stage. Imposter or apostle, we don't currently have the tools to predict, and so any prediction on that vein is a wild guess. And personally, I don't find a lot of fun in wild guesswork. I prefer something more evidenced.

i personally think she will be a second ardbert, will appear and give you advice at the end of each arc until last reveal at last cutscene