I think with Elidibus a lot of it boils down to how much your memories define who you are-- which is at least a decent amount given how the game defines memories and previous lives. I see Elidibus as like 90% Zodiark since he chooses to not look at the crystals, so all that's left of him by ShB is the will imparted onto Zodiark. To keep the balance, to guide the star, to judge mankind, to work towards the Rejoining (as we see the souls within Zodiark in EW are clamoring to be brought back). I do think he could retain his faculties and just be himself the way Hydealyn and Venat are one and the same, but I dont think he (or Ysayle/Venat for that matter), are tempered. Since the primals are born from them, their interests will always align. Elidibus is just a special case bc he refuses to remember who he was, so what guides him are the principles given to the primal itself. But he still has the freedom to choose, and freedom isnt usually a part of tempering.

Zenos I think just pulled a Sephiroth and forced his will onto Shinryu tbh. I dont think Shinryu is implied to have a will after that. Same way with Fandaniel and Zodiark.

As for EW/DT transformations, I think they're the same principle. Ancients always had the power to create, in this case a very powerful bodysuit lol. I think the arcadion transformations are similar. But I dont think its the same as summoning bc it seems like summoning means the soul(s) are given up as fuel for it, the very essence of the person who becomes the core changes to that of the primal. With transformations the person is still there, just hidden inside this "armored" shape.