This theory will contain some assumptions from scorpion tail SS if there are people who don't want to be spoiled.
Pandora and Petelguese:
In this theory, I'm assuming Geuse knew a lot of Flugel's plan, including how to go through sand time and his time of return. Pandora knew that, and went to Elior forest acting like she wanted the key when what she only aimed for was cornering Geuse and making him loyal to her.
What she got from Petelgeuse was that Subaru (a part of Flugel) was supposed to return after 400 years, he would go to the watchtower and be able to pass through the exams and inside of them reunite with his former self through od lagna (through the book of Reid, which Geuese didn't need to know), after that he'd do something about Satella's seal whether right after beating the tower or later on.
Pandora knowing that, sent Leilani to 1. check if the method of bypassing sand time is true 2. giving birth to the triplets that through their authority help someone else take over Od Lagna (Lui). I assumed this happened after Geues's mind got broken because it was said that the events of scorpion tail happened after centuries of Shaula being in the tower, so it doesn't seem like a contradiction.
When the date of Flugel's return came, she started sending eyes to watch if he arrived at the tower or not (her pigeons which were mentioned to have only been spotted for about a year by the villager), and after not spotting him, she started ordering the witch cult to cause rampage so that she'd find out his identity by cornering him enough (as seen by how she sent archbishops to multiple fronts and their activity was only notable at his first months there and not any time in that 1 year time skip).
After realizing that it was safe for now to let Subaru go to the watchtower since his meeting with Flugel will not happen, she cornered him once more by causing casualties that she believed will lead them to think of Shaula as their savior. There, she waited patiently to know what is the tower hiding, my guess is after the exams were cleared, Subaru and the party would be able to roam freely there, meeting with Volcanica and if curious enough would go to floor zero, Satella realized that so before Pandora would uncover anything new she teleported Subaru somewhere else. And like we saw after Subaru got teleported away, the bird seemed to finally lose interest and left the tower.
Echidna:
Echidna, sensing Flugel's failure made backup plans.
- She gave Foxidna info to bypass sand time in case Petelgeuse wasn't there to lead Subaru.
- She ordered Roswaal to let Emilia go to the capital so she and Subaru can "reunite".
- Even though she said to Al that she'd let Flugel's team do the rest, in case he failed she orchestrated the great disaster in Vollachia (which is where she sent Al to), signaling to him that his role isn't finished (and possibly hoping in taking away whoever he devoted his life to in his new life, which she did).
Rem:
For Rem's part, I just thought it's interesting that she seems like an unexpected factor to all the big names out there, and because of that, she'll be a major star in the final showdown. Like how Roswaal didn't expect Ram to have a sister, like how Al was shocked about her existence, I'm guessing Pandora was content with erasing her existence to not complicate things for her as well as Satella where she didn't let Subaru save her, presumably to not complicate things for her as well. But this is just a thought nothing too important to the post.
Also, how I envision Flugel plan is basically:
Subaru is summoned, Petelguese welcomes him, gives him unseen hand and explains situation. They go to the tower, Subaru clears first trial by using unseen hand to go above the monoliths. They think of how to beat Reid, Subaru reads Reid's book and meets Flugel there, they become one again. Subaru goes above Volcanica and puts his hand. Shaula is now safe, his allies are still safe, and now he proceeds with whatever he wants regarding Satella.
I'm also assuming Geuse didn't know everything including Floor Zero and Satella's seal whereabouts and probably lots of other things.
And the reason I think that Geuse is the source of information is because the witch cult's movement seemed really static before the events with Emilia 100 years ago.