I think we just have different interpretations of certain scenes that lead to a slightly different view of the whole (kind of ironic considering the scale of the ensuing debate.) I agree that the contents of the plan were pretty much entirely Lahabrea, but I think Elidibus did far more than stand on the sidelines, occasionally saying "Hey. Don't f[KUPO]k up." (Which he also did, don't get me wrong.) I could be over-estimating his involvement, but it hardly seems limited to oversight, imho.
It looks to me like your soul borders need to be fuzzy in order to summon a primal into yourself. We haven't seen that as something within Lahabrea's power to grant, while the game went out of its way to imply that Elidibus was capable. That, right off the bat, primes me to look for his involvement (and, if false, is the source of my bias). They seemed to be working different angles up until the last scene of 2.3 (more on that in a moment), but from then on, at every turn they're shown meeting privately (and I wouldn't be surprised if they were meeting privately prior, off-screen) until it concludes with the ominous foreshadow of them both behind the throne, so I keep seeing things through that lens.
Here's how I interpreted the same scenes you mention:
Nabriales (later shown to be following Lahabrea's lead) questions Elidibus when he meets with Minfilia and the Warrior of Light because "our plans are already in motion; your intentions are unclear" and he assures them that "we labor not at cross purposes".
When Ramuh not only falls to but accepts the Warrior of Light, Emmeroloth and Altima concede that the "potential of man" is limitless. English is (oddly) the only language where Altima doesn't explicitly say, "It's exactly as Elidibus said." Elidibus, meanwhile, stands center stage and states that while the Warrior of Light is getting too strong, it's at such great cost to Hydaelyn that they only need to "nurture the strength of the gifted to forge the final key." Hydaelyn's children have survived the Rejoining seven times, their souls can't withstand more. Out of left field, Lahabrea makes his return and and backs him up, letting everyone know that it's already being worked on; a new deiform will rise. (Summoned by one of Hydaelyn's children to be fought by one of Hydaelyn's children.)
Lahabrea succeeds with Ysayle (which angers the ambitious Nabriales, causing him to go rogue as soon as he sees a chance to rise above Lahabrea), and as soon as he (who is not of this world and has not partaken of Zodiark's power as Lahabrea has) gets himself killed, the first thing that happens is Lahabrea meets with Elidibus.
Not only are they "of one mind" that they should be working faster, Elidibus is confirming with Lahabrea "So, about the next Rejoining. Your plan is for the chaos to start in Dravania and Abalathia?" (Hell, in French he even says, "Your plan is perfect.") The sole point of contention, that I can see, is when Lahabrea follows it up with "Yeah, baby! Primals! This is gonna be great!" and Elidibus reminds him "The means are not the end. Zodiark is the end." Lahabrea no longer treats his plans as "only what is necessary for my god to be reborn", but seems more interested in the plan (giving rise to primals, manipulating how and where they form, toying with the Warrior of Light) than its purpose.[L] Nevertheless, it concerns me. They have...extinguished that which should rightly be eternal.
[E] Mayhap he was not wholly mistaken. Greater haste may be warranted.
[L] We are of one mind.
[E] The northern lands, then?
[L] The earth is fertile, and the seeds well sown. By my will, they shall reap salvation unlike any the world has known.
[E] By His will.
The Eighth Rejoining - hard stop. How it happens doesn't seem to matter to him as much as that it happens. When they meet about Nabriales' death, I see him as telling Lahabrea, "Your plan has my support, but don't forget its purpose."
Why would Thordan awakening the triad be part of this plan in the first place? Thordan was going to destroy every dragon, every primal, every threat to the peace. Consuming the Triad into the boundless wellspring of the Eye to sustain his coming wars isn't so much "awakening" them as taking them off the board entirely.
I don't think he's displeased that Lahabrea acted, just that he failed. No dead Warrior of Light, no all-powerful Thordan, no confluence of chaos in the war-torn north, no Eighth Rejoining, and now the Triad are unconsumed, unguarded, and waking up just as everyone and their mother (including Bismarck and Ravana, apparently) are making a play for their power. "And now it falls to me to deal with the consequences."Originally Posted by King Thordan
Granted, none of this proves Elidibus had anything to do with God-king Thordan, it just imho defends it as an interpretation.