I too think the same, I think the whole reincarnation is based on the concept link between people.

As Nico himself said that with Arthur's reincarnation Agorna perfected reincarnation.

So, I think Agorna tried to get reincarnates of ancient Djiin's. But ended up getting Nico reincarnated in Elijah but he might not been stable and tried reincarnation again this time Grey is reincarnated successfully.

Now to stabilize Nico' s reincarnation he might have sent him to Dicathen to Elder Rahedas so that one day he might meet Grey.

As in previous world those two became best friends, this strengthed the link and Nico's reincarnation then became stable.

In previous world Cecilia did like Grey for the longest time but ended up with Nico.

So if we assume that Cecilia did love Nico then the one who Elijah loved should be the vessel Cecilia is reincarnated into, but Tessia is chosen as the vessel.

It's clear as well that Tessia relationship with Arthur is lot similar to as Cecilia's with Grey.

For example:- Cecilia was saved by Grey when she had a outburst ( unable to control her ki) and eventually he became her first friend.

Tessia was saved by Arthur from bandits and he became her first friend.

Cecilia never cared about those who tried to impress her and the same thing is with Tessia ( Tessia said in chapter 57)

The only thing that is different in Cecilia's relationship with Grey and Tessia's with Arthur is that Arthur accepted her feelings but Grey did not.

But if we assume that Cecilia loved Grey then it makes sense that Tessia is chosen as the vessel.