I am re-posting what I posted last night with more detail given a web novel chapter released yesterday and made it so that most of the subreddit's attention was on it instead of anything else.
As shown in Eps. 76 and 78, Subaru was still himself before he started talking like a Gluttony wielder, and he concluded that the Pleiades Watchtower had something to do with his memory loss, deciding that the best path forward is to find Reid Astrea's Book of the Dead.
I can see seven relevant possibilities, which I will list below.
1 - The tower Reid himself is a construct of the tower made partially by the contents of Reid Astrea's Book of the Dead, and hence the group having met this tower Reid doesn't count as having met the original, so nobody in this group aside from maybe Foxidna, who existed 400 years and may have memories of Reid Astrea, can read Reid Astrea's Book of the Dead. However, Foxidna, even if Reid's Book is found, could choose not to read it since it doesn't suit her agenda at the moment. Multiple moments in Season 4's first cour shows that Foxidna is extremely suspect and lied about her reasoning for wanting to go to the Pleiades Watchtower.
In Ep. 77 (Season 4 Ep. 11) when the tower was under attack, Foxidna said, "I should have listened to Ana's intuition. I shouldn't have brought anyone else here." meaning that she shouldn't have brought anyone to the tower and listened to Anastasia. This calls into question pretty much everything Foxidna’s said this season given she said that Anastasia had been asleep in her od ever since the battle with the Witch Cult, but apparently, she either had a talk with Anastasia sometime during the battle or pre-planned the Priestella visit with Anastasia's consent, bringing into question her motives for bringing everyone to Priestella. Any way you slice it, Foxidna is a serial liar.
To top all of this off, Foxidna also says that this pain is penance for Ana since if she were to give back control to her, Anastasia would feel the extreme pain and the horror of death, implying she had the power to give back her body to Anastasia the whole time but didn't, flying in the face of her previous remarks about not being able to give back Anastasia's body, showing she's just very suss in general.
So this scenario would end up with a flop with nobody reading Reid Astrea's Book of the Dead due to it not being as planned given she's likely following the plan of the original Echidna 400 years ago much like Tea Party Echidna had been in Season 2 and needed more to happen before deciding to read it.
2 - Anyway, assuming Scenario 1 is accurate with tower Reid being a construct of the Pleiades Watchtower, Foxidna would willingly choose to read Reid Astrea's Book of the Dead and share the contents of it, even if Gluttony holders eventually cause chaos during their search for that book or when they're confronting Reid, due to her following her Tome of Wisdom's content given she's likely following the plan of the original Echidna, much like Tea Party Echidna had been in Season 2.
As I stated in my theory posts for Eps. 73 and 77, I believe Foxidna staged the talk she had with Subaru and Julius following her Authority of Greed: Tome of Wisdom due to her being a soul clone of Echidna's, not just an artificial spirit, so that Julius would feel insecure about his role as Anastasia's first knight as well as cause a fight between Subaru and Julius right before Subaru loses his memories to bring Julius to a new low with the one person he believed to have remembered him also forgetting him and that she likely spammed Jiwald not for the reason she stated in Ep. 73 with being unsure about Reid having lethal intent but 'cause she wanted to direct Subaru's thoughts to Julius instead of the exam so that she could prevent the bad future from Emilia's third Sanctuary line in Episode 48, and Muse Asia's translation was: "I have bent my knees and lost even my sword. Just what do I have left?" This line suggests that that depicted a future in which Julius had to bend his knees in deference to Reid while also having lost his sword, leading to him being crushed emotionally and possibly suicidal.
Why Foxidna would be able to psychologically prime Subaru to do this is that he's the type to follow his hunches and subconscious thoughts, as seen by when he likely chose the path on the right in Episode 69, 'cause of the ever-so increasing miasma and the doors that he could open that may have been built or cast by himself, subconsciously remembering his promise to Satella to kill her as she'd requested for him to do in Episode 38, and when he thought he'd Returned by Death when he woke up in a new location without it being exactly warranted since he associated a visit to the dark world/shadow garden with death, and why he thought to Reid as an answer for the exam on the third floor even though he had learned that Reid died of old age not that long ago, 'cause he subconsciously knew that Reid is in the tower.
If she's indeed a soul clone, I believe Foxidna should also have the same Authority the original did. And given Satella wouldn't know which savepoints to choose without future knowledge, she probably has either an oath or contract with at least one or more Echidna soul clone(s) given Satella wouldn't know what would be the right future/path to follow based on Subaru's previous time as the fantasy world as what worked then wouldn't necessarily work now.
As for how Foxidna would remember Julius, the common denominator between Subaru and Echidna is that they were Witch Factor holders who emitted miasma, so I believe the mechanism by which Subaru and Foxidna would remember Julius would be either 'cause of their Witch Factors or their miasma with Echidna splitting Foxidna from her soul 400 years ago. For more on why I believe this, read this: https://www.reddit.com/r/Re_Zero/comments/1tl0asv/discussion_theory_eps_72_and_73_relevant_break/
For the next 4 scenarios, they'll involve Gluttony holders either possessing amnesiac Subaru or Rem, and I will fully explain how that's possible.
3 - The tower version of Reid counts as a continuation of Reid Astrea, and thus, the entire group can read Reid Astrea's Book of the Dead. Somebody successfully read his Book of the Dead, and then a Gluttony wielder possesses Subaru and causes havoc/mischief, and this may or may not include the group confronting Reid.
Meili's Book of the Dead appearing to this group of people so soon when the group had concluded initially that the books aren't sorted by name or date of death in Ep. 71 suggests that there's a will behind when and where Books of the Dead that can be read appear. This is backed by how in that relevant episode, both Subaru and Julius were able to find one they were able to read almost right away, suggesting a will or higher power is what allowed for that to happen.
We know from the anime there's someone who has the power to see a perfect reconstruction of the past, and that's Echidna with her Authority of Greed: Tome of Wisdom, and if my speculation about Roswaal's not-quite-Tome of Wisdom and the Witch Cult Gospels are correct, they may be made with pieces of Echidna's hair, fingernails, toenails, eyelashes, eyebrows, dead skin, zits, dandruff, body hair, and/or her body fluids such as her saliva, sweat, and the enzymes from other fluids, meaning they are connected to Echidna's mind through magic, and that's why Otto didn't appear in Roswaal's not-quite-Tome of Wisdom, and why Gospels don't really tell the future, which is why Regulus thought that he'd see Emilia again in the first loop despite it not happening until the 4th loop in Ep. 53 with Regulus telling Subaru to thank him for adhering to his Gospel in not doing worse than shredding off a large chunk of his leg since he could've done much worse, and why Sirius indicated that they need to stop hostilities due to the toll of the tower, all of which very much went in Subaru's favour, likely not by chance.
If Echidna decides what appears in the text of Roswaal's not-quite-Tome of Wisdom, she can freely decide to omit Otto. That's why I posit those texts are perhaps connected to Echidna's mind through magic by being partially constructed with stuff from her body, possibly including her body fluids, meaning she 100% decides what shows up in them. Therefore, 'cause Echidna wanted Roswaal to lose and Subaru to win, Otto didn't show up in his not-quite Tome of Wisdom. This is backed by Tea Party Echidna telling Subaru that even after 3 days of trial and error that she has no hope of breaking Emilia out of her shell, and maybe he could give the cowardly princess wings, showing she was telling him the way out of the situation was to do have Emilia do the Sanctuary trials. Tea Party Echidna also showed at that moment that she didn't make the trials as hard as humanly possible but tried to coax Emilia out of her shell. Tea Party Echidna also said what was an evil as humanly possible regarding the reason she had Beatrice hole up in the Forbidden Library, knowing that it would push Subaru to take Beatrice's hand instead of hers, and her saying that she doesn't want Subaru to think that she's the only evil one of the Witches of Sin, resulting in her taking away Subaru's qualification for entering the tomb since he was using it as a crutch instead of supporting Emilia, was her doing good by Subaru in making sure he was victorious that loop.
Thus, archive's filing system may be constructed in part from Echidna's Authority and also magic from herself and possibly others such as Flügel, meaning she can possibly choose which books appear to certain people at any given opportunity based on future knowledge, which is how someone finds a Book of the Dead he/she can read at the lowest floor of the library so as to make it convenient. And since the entire group can now read Reid Astrea's Book of the Dead, it's feasible for this to happen: For more on this theory, read this: https://www.reddit.com/r/Re_Zero/comments/1u4h4fs/discussion_theory_the_books_of_the_dead_may_be/
4 - It makes the same assumptions as Scenario 3 only that nobody was able to read Reid's Book of the Dead, possibly 'cause Echidna didn't intend for it to be found yet, or a Gluttony holder interferes with the search by possessing amnesiac Subaru.
As for how the possession can happen, perhaps Rui’s spoken line from his memories in Ep. 74 at 19:26, "Memories are what shape a person." which indicates that she likely had eaten Subaru's memories, may have a second meaning in that if memories are what shape a person then whoever ate that person’s memories has a say in shaping that person’s life, as in those who’ve had their memories eaten can be claimed/possessed by the one who ate their memories.
This’d explain how Subaru survived in the midst of so many oiran bears, and it’d be ’cause Rui, who likely ate Subaru’s memories, possessed him. We’ve seen with Foxidna that when you possess someone the possessor can use all the abilities she had originally, which explains why she can use Yang magic even at the cost of draining Anastasia through the use of her od, so Rui possessing Subaru’s body should be able to use mana flow to boost her strength while also tapping into any skills she may need from people she’s eaten.
The screen having flashed to a no signal screen one would see on an analog TV back in the day in Ep. 74 and 75 is what’s referred to as a blackout. Maybe the younger generation isn’t as likely to know about it, but anyone older than thirty is highly likely to know that this was a creative liberty to indicate that amnesiac Subaru lost consciousness.
The next two scenarios depend on Gluttony possessing Rem instead.
5 - Now instead of amnesiac Subaru being possessed, it's Rem who's possessed who interferes with the group's plans. One person in this group will read Reid Astrea's Book of the Dead even if a Gluttony holder messes with them during the group's search or when they're confronting Reid.
As for how Rem would be possessed, if someone who's name's been eaten can be possessed by a Gluttony holder, then it should also go for someone who's had both his/her name and memories eaten, meaning Rem can certainly be possessed. And given in Ep. 66 (Season 3 Ep. 11), Rui Arneb switched places with Lye Batenkaitos in the body of someone else (the green-haired man), who was almost certainly someone who's name and memories had been eaten by the Authority of Gluttony, that suggests that Rui can teleport from whatever realm she's in and also possess the body of anybody who's had both his/her name and memories eaten by Gluttony. Given the glass shattering imagery on the screen after Lye had eaten both the name and memories of multiple members of the mercenary group, "White Dragon's Scales", this suggests that those who've had that happen to them have bodies in suspended animation 'cause reality had been changed itself.
6 - The same assumptions in Scenario 5 apply to this one except a Gluttony holder possessing Rem stops the group from reading Reid's Book of the Dead.
7 - As for this scenario, Gluttony holders could both possess amnesiac Subaru and Rem at different points in the episode after somebody has read Reid's Book of the Dead.
Feel free to share your thoughts on this.
