I'm trying to remember what all Elidibus has even done, if only to try and see how well those actions fit together as parts in either of his stated/implied goals. What I've got so far:
He attempted to possess Minfilia.
Had he been successful in doing so without being seen, he could have manipulated the WoL incognito. Even having been noticed, successfully doing so would've given him more leverage over us than he ended up with.
He might have sent Cylva over to the First to manipulate Ardbert and Friends into setting the First up for a Rejoining.
Obviously this benefits the Ascians' goals, but I can't recall if this was specifically attributed to him. I'm also not sure if it was Elidibus who did this, or Mitron and Loghrif.
He brought Ardbert and Friends over to the Source to deal with the WoL.
Ardbert and Friends believed killing the WoL would trigger a Calamity, so perhaps Elidibus believed this too.
He reached out to Urianger, who then plotted to send Minfilia to the First.
In direct contrast to the previous plan, this stabilized the First's consumption by Eden/The Flood of Light. Hedging his bets?
He might have set Unukalhai up to help us deal with the Warring Triad.
Apparently Lahabrea's antics from the 7th Calamity and possibly onward were tipping the Source's balance too far in favor of Darkness. Possibly an attempt to fix that?
Speaking of Unukalhai and Cylva: He saved them from the Thirteenth's destruction, though their bodies died in the process.
Setting up pawns for potential plans in the future, natch.
He tasked A&F with retrieving Nidhogg's Eyes, then gave them to Ilberd.
Shinryu would easily serve the twin purposes of provoking a new Calamity and/or killing the WoL.
He possessed Zenos's corpse, then manipulated events in Garlemald.
This chain of events led to open war with the Eorzean Alliance. Presumably he knew about Black Rose and its potential.
He tried to 1v1 the WoL in Zenos's body and failed.
No faith in Black Rose to do the job, or just trying to get the WoL out of the way when the opportunity presented itself?
He stuck around in Garlemald until Zenos arrived and kicked him out.
Sticking with the Black Rose plan after all?
He sulked on the Moon.
Even the Ancients need to cool off with ice cream and soaps when nothing is going their way.
He possessed Ardbert's corpse to manipulate the masses.
If there was a goal to this beyond exploiting the First's hopes and faith to become stronger, we'll never know.
He revealed the truth about the Echo's origin.
This might be tied back to one of his earliest statements about the Echo, and how the WoL and/or Minfilia would be of one mind with the Ascians. But by this point he's firmly entrenched in his stated goal of killing the WoL.
He didn't need to tell us, so why?
He attempted to possess the Exarch.
As seen by the end of 5.3, getting access to Syrcus Tower gave him a tremendous shortcut to power.
He abducted, but didn't kill, Y'shtola.
It's hard to believe he couldn't have simply killed her, so what happened here?
He tried to Myste us to death, and failed harder than Myste did.
I can't even recall what the point of this was. The whole thing felt like the man himself didn't know, either. It ended with an obvious attempt on the WoL's life, more admissions of things he didn't need to admit, and then...
He attacked the Exarch, stole his soul crystal, and used it to hijack Syrcus Tower. Sent a horde of half-summoned heroes from across time and space to kill the WoL. Then he hoped himself into a Super Saiyan.
Trying to kill the WoL, natch.
If there's anything I missed, feel free to let me know. I've not gone through the 7th Astral Era quests in a long time.
As for the idea of exploring Past!Amaurot through deep memory manipulation or some other means, I'd be all for it as a one-off, or even a several-off thing. Rule of Three and what not. Short of Hydaelyn regaining her voice and dropping pretense, I doubt we'd be able to learn much more about that period of time otherwise. I can't see such a concept carrying an entire expansion, however. The concept would probably wear thin after 2-3 zones if there wasn't something else to break things up. Maybe one smaller zone without gathering nodes, 1-2 dungeons, and a solo duty?