Some thoughts regarding Ella's goals.
At first i thought Ella's plan was to find a suitable body for a merge of 3rd and 4rth since it seems apostles came to be in pairs, but those were imperfect forms lacking the power the single entity should have, like authorithy and power combined, or Memory and Body, but now things got messier.
Eye boy does seem out of this plan, after all it wants the powers of all apostles, my guess is that it is the thing who originated all apostles/arbiters in the first place and now it wants to be complete again.
For a long time i have a theory that the combination of 3rd and 4rth powers is capable of copying other powers, this is what Ella wanted but after her 5 evolution she didn't develop memory powers, Mc did since his 1 evolution.
I think that she wanted either someone capable of controling everyone thus having acess to all powers through command (Jake) or someone capable of manifesting all powers (herself or Mc)
Maybe her plan is to absorb the one who fits that, thus explaining why she didn't want it to be Mc, the hearts might be exacly for that, to gain the powers of monsters
And for what end, i guess she knows only power can protect humans and monsters from the bizarre shit across the universe, she knows terrifying things are lurking out there.
Also the twins thing might be her trying to manifest arbiters, transforming people from pairs of apostles so they can become one, now if she wants them just to be around or to try and abosorb them i have no ideia.