First of all, this is all brilliantly thought out and I'm so excited to be reading it.
Secondly, I've got a number of theories on this, actually, and only a few of them will have me admitting that I'm just on copium. My goal is to keep this as concise and straightforward as possible, but I'll be honest it's 2am and I'm not sure how well that's going to go at this point. Regardless:
It's clear from a diary perspective that we're leaning further and further into whatever the hell was going on in the Halazia MV (the most unexplained and befuddling MV of all time from a lore perspective, my beloved). The name drop of the Priests of Halazia, the acknowledgement of Sopro, and the resurgence of World Z conversations are leading us back in that direction.
Now, as far as I can remember, all language surrounding the Priests of Halazia has been past tense--whether this is because the order didn't really exist anymore by the time our boys made it to World Z or because the topic at hand was focused on the original priests is hard to say, but it does feel like if they were a current and active organization, Left Eye would have used present tense. This brings me to my first theory --
World A Ateez will bring back the era of Halazia in their dealings with Sopro, and with it, its priests. This is, of course, predicated on the idea that they ultimately return to World Z (please please please) in some form or fashion, which I think is extremely likely both from an overarching storytelling point, and with the supplementary evidence of the "Your Thunder" letter being released with The World to the End. Furthermore, I think that searching for the best way for them to subdue/contain/deactivate Sopro will probably lead them back to World Z in some way.
In this case, I think that the ghosts from the Gayo Daejeon could be the spirits of previous priests watching them as they interact with Sopro. Maybe they'll receive some sort of interference or guidance from said spirits (or potentially intervention in the form of Halateez? This is like 90% copium though) in the upcoming diaries as a result.
I've spent a lot of time wondering how they'd get back to World Z without the Cromer, but if this sun artifact is the counterbalance to the Cromer, then that potentially answers that question. And with Yunho in archaeology and the introduction of World A's version of Left Eye, I think that the sun artifact could potentially be a pretty reasonable solution to the Sopro issue overall.
My second theory is a little more substantiated by the end of the In Your Fantasy Diary (all diaries can be found here).
Yeosang: “It didn’t disappear, it was just hiding!”
Yeosang turned his tablet to the members. On the screen was a live clip of a video creator searching for true ghost stories. He talked while walking through a graveyard. A statue that didn’t exist until yesterday suddenly appeared overnight, and monsters were rising from the ground. The bust statue, made of corpse, bone, soil and grass, had grown into a human shape. With a shriek, its eyes shot open. It looked enraged. People watching the live video called it “the resurrection of Frankenstein.”
Not long after, a war broke out over there.
From an extremely literal and lore-centric perspective, the ghosts in the recent live performances make a lot of sense here. As does the return to the Kingdom version of Wonderland -- the lore drops on Kingdom were some of the most blatant and simultaneously best obfuscated. As an Atiny who didn't show up until well after Kingdom was done, the hints were obvious to me from the get-go, but for Atiny who watched those performances live, the lore was right there but just out of reach. I can't speak for anyone else, but it truly does feel to me like Ateez are getting back on their lore focus with performances, and using Kingdom Wonderland (and Hala Hala, and the OG Wonderland, and other throwbacks) feels more than a little bit like a batsignal.
With that said, we don't actually know what Sopro's been doing in its time underground, or what the monsters and bust statue are in a literal sense. But I think that San's performance of Phantom could represent Sopro's recent experiences in hopping from person to person, gaining a large amount of power, being pressed into the earth, and then coming back.
As a brief side note: the fire he puts out could serve as potentially either a nod to the sun artifact as a way to help neutralize Sopro or a nod to Sopro itself. I'm more inclined personally to connect it to the sun artifact given the addition of the lantern in the Gayo Daejeon, inclusive of the hourglass-not-hourglass structure within it. (It's just entirely too hard to tell.)
I've now been working on this for entirely too long and really need to sleep so I'm capping it here for the moment, but I do plan to return to this tomorrow when I have time to clarify anything that needs it (sorry) and continue my thoughts with further fervor.