The dev. team wants you asking precisely these questions.
You can never count on narrative points to be true. Sometimes we're lied to. However, there are a few considerations that stick out to me more than others (until they're disproven, at least). A lot of this information you probably already know but it helps to build a strong analysis:
#1) The end-all, be-all raison d'être of the Ascians is the second coming of Zodiark
Every Ascian we've ever come across has been acting in service to Zodiark's resurrection.
The way the cosmos works, you can somehow get into the aetherial realm by passing through the planet, like the corporeal is somehow a projection of the aetherial. I don't really understand that part, but it is what it is, lol.
The will(s) of the universe, in the aetherial realm, either inherently or through eventual division became aspected to Light and Darkness. That's the "energy spectrum" they exist on. In time, the will of the Dark accumulated more power than the will of the Light. Hydaelyn claims that Zodiark was simply greedy and desirous of power over Her (as it is in the Dark's nature to be so). She cast Him out of the aetherial realm, through the planet, and into space, where He became the moon. She claims that this was necessary self-defense.
This essentially imprisoned Zodiark; He is cut off from the aetherial realm and unable to exert influence on the cosmos. (Hydaelyn's own actions are likewise limited, though not as severely.) However, this act put such stress on the cosmological foundations of the universe that it "cracked", and like a broken mirror, cast thirteen additional reflections of the original image and scattered the Light and the Dark's remaining energy into those worlds.
The cracks in the mirror became stable, nigh-impenetrable barriers between independent, overlapped dimensions. This division and isolation of Zodiark's energies, combined with his imprisonment, have rendered Him powerless. However, acts of extreme chaos and aetherial imbalance cause these membranous "cracks" to buckle, collapsing the "Reflected" dimension back into its "Source" dimension and sending all of its aether (allegedly) back into the aetherial world. When enough of it has accumulated, it should theoretically be possible to resurrect Zodiark and circumvent the aetherial rift He is presently trapped within.
The Ascians thus seek to blow out the dimensional barriers by causing calamities, each one further weakening Hydaelyn and empowering Zodiark (somehow).
#2) The fourteen black-robed Ascians and Elidibus are on different paths.
When Elidibus makes contact with the Warrior of Light in 2.1, it sends a wave of concern through the black-robed council. Nabriales is the one to confront him, pointing out that their plans are already in motion (manipulating the Warrior of Light into inspiring more powerful primals, inspiring new primals, merging man and primal essence to escalate conflict, steering the beast tribes towards a new god) and that Elidibus shouldn't be moving unpredictably and without informing them. Elidibus counters that they have their mission and he has his own, but they're both working in service to Zodiark.
After the fall of Ramuh and his flat out assent to the Warrior of Light's path, the council starts to take Elidibus more seriously, affirming his observation that the stubborn survival of Hydaelyn's chosen and the stability of the walls of the soul in the face of seven dimensions collapsing is noteworthy and that Her champion might be forged into the final key for their plans.
#3) The only motive that matters is Elidibus's.
The black-robed Ascians have proven to be unreliable at best. They are not only working in service to the Dark, they're consumed by it. They show the same greed, instability, and pettiness that Hydaelyn accuses Zodiark of. There are strong hints that each Ascian overlord was once of the dimension it is tasked with overthrowing (thus Lahabrea's importance, being from Zodiark's original dimension), so perhaps they are corrupted mortals, and thus are the idealized Dark side of the heart of man.
Take Igeyorhm, so zealous to prove herself and fulfill her mission that she destroyed the dimension she was meant to deliver to Zodiark. Take Nabriales, so determined to undermine and outshine Lahabrea that he went rogue in the middle of an operation and got himself killed. Take Lahabrea, whose arrogance and desire to sit at Zodiark's right hand led him to be outsmarted by mortals, thus robbing the Ascians of their connection to Zodiark's power through the Source dimension and forcing Elidibus to fill the vacancy.
At the end of the day, Elidibus is, among his other roles, their babysitter. He ensures that neither the Dark nor the Light energies at work gain enough traction to destroy the dimension meant to empower Zodiark.
The black-robed Ascians are thus perhaps little more than pawns. They are Zodiark's puppets, acting out His impulses for rebirth and revenge. Notice how Lahabrea slips back and forth between espousing Elidibus's "reunion, restoration, preservation" narrative and just yelling about how Hydaelyn has to be "burned out like a parasite" and that Darkness shall consume all Light and reign forevermore. Nabriales shouted similar things as he invited the powers of the void to assist in the destruction of aether aligned to the Light end of that energy spectrum.
Lahabrea also gave away more than he knew in the Aetherochemical Research Facility, pointing out that when we forcefully separated him and Igeyorhm back into individuals, he saw this as the restoration of Hydaelyn's bondage, from which Zodiark saw him freed. The soul is a "wall".
In babysitting the master plan, Elidibus is able to weave a strong narrative to justify his actions. An observer will note that he sometimes works against the Darkness, which allows him to claim that if he works against the Light, he must have a good reason. An observer will note that he always works in the interest of balance, which allows him to claim that his allegiance is to the existence of the planet itself.
It's a compelling case, on the surface.
He's just out to heal the universe, right? He's just out to put things back on the right path, right? And that's even just accepting everything we've been told about Zodiark. What if Zodiark's actions and motivations have been cast in the Light of those who cannot understand His point of view? What if He's not so bad? Hells, maybe He is, maybe the Dark is a lot of things we think are negative. But what if they're necessary? Elidibus is just looking out for the best interests of the health of the cosmos and the continued existence of the star, right?
#4) Then what's with all the smoke?
Everything Elidibus has ever told us has been calm and logical, and moreover when he gives you information about the world and its history, it always turns out to be true. But it also continually turns out to be the case that it's not the whole story, and the parts he left out just happen to change the context of the whole.
Everything Elidibus ever told the Warriors of Darkness was true, he just led them to believe that their only options were void or rejoining and urged them to choose the "right" path, not letting all of those deaths be in vain, to rejoin their world's spiritual lifeblood to the Source and one day be reborn rather than becoming twisted, starved abominations incapable of knowing life or death again. The efforts of one scholar making tough decisions were able to save the First, Elidibus just happened to leave out that it was possible because he wanted to, because it served his agenda. He had no love for them or their world, he cast them aside the instant they were no longer useful. Even the Griffin mocked him for it.
What will be Elidibus's stance on balance once Zodiark has all the aether he needs to be resurrected?
What place will mortals have when the world is restored to a form that never knew their kind?
How does Zodiark really feel about all that is and all that has happened?
#5) I'm biased as all hells.
To the seas She cast Her children, for fear of the Moon
For hate of the Star, to the seas he cast His doom
I don't trust him for a second. I'm willing to eat my mortarboard if this turns out to be wrong, but if Elidibus is Minfilia's equivalent, then he's not out for balance. He's possessed of the will of Darkness, he is the Word of the Father, and he is a snake.
I don't even know if this opinion is a popular one, but I would be remiss to not admit my bias, lol. Besides, what if Elidibus is a snake, but more in reference to Ophiuchus and Serpens than just The Deceiver? What if Zodiark isn't terrible, eh!? Ohohoho! Loophole! /clenches fist
My (current) dream ending:
Elidibus is the true antagonist and the defeat of the recently-reborn Zodiark only causes Elidibus to become freed from his tethers and become even more of a threat. I want to see him embrace all of the things he's ever claimed to be, which I presently believe are a manipulative cover. I want to see him attempt to collapse the remaining dimensions and the Source itself (See: Time Compression) into a new Genesis in his own image.
He'd be "right" to do so; there's cosmic "justice" in it. He'd undo all the damage done to this world, preserve the existence of the concept of the universe, the idea of the cosmos, the core reality of it being a thing at all. He'd snuff out the divisive world souls and their petty war and ensure that Creation, even rebooted, was preserved and thrived. He'd be aghast that we still defy him; men throw away their lives for less righteous causes every day, why do we stand against him NOW?
Because the entire point of Final Fantasy is the best humanity has to offer stubbornly fighting for the preservation of our kind and control of its destiny. If that means killing entities intended to guard the planet, we do it. If it means killing our own gods, we do it. If it means tearing down the foundations of "natural order" and living out the rest of our lives in a beautifully broken cosmos where Life as we know it thrives, we do it. When the credits roll, the reins of history in the hands of man, for better or worse.
That said, by all means debate me into the ground and tear that apart if it makes for a good conversation!
/makes drinks for everyone