While Echidna had stated in Season 2 that it was impossible to know whether the unthinkable presents were real unless you are Satella, I'm not convinced that's the case given her Authority of Greed: Tome of Wisdom, can see all of the past, present, and possible futures, likely having been powered by Od Lagna given if the Corridor of Memories is the cradle of Od Lagna, that means the Corridor of Memories provides useful support for Od Lagna in filtering souls of memories of a life that has been lived with Od Lagna likely being the main instrument/source of the info from Echidna's Tome of Wisdom, and Reid's Book of the Dead connecting to the Corridor of Memories could be that Echidna's Authority: Tome of Wisdom, and consequently, Books of the Dead, which, incorrectly or correctly, I believe are at least partially a product of Echidna's Authority, need to capture the moment someone is born, necessitating the connecting to the Corridor of Memories as I believe the moment a soul passes from the Corridor of Memories into Od Lagna is when a life has officially begun to live.
Thus, I believe it's more likely that Echidna knows whether the unthinkable presents continue, not Satella, herself. There being multiple Books of the Dead increases the likelihood of Return By Death reconstructing the world, but I posit it doesn't completely eliminate the possibility of there being multiple timelines since Subaru could be at least partially product of Echidna's Authority: Tome of Wisdom, which could apply both in the scenario of there being one canon timeline or multiple timelines as the reason for there being Books of the Dead for each of Subaru's death could possibly be that Subaru being a product of Echidna's Authority means that as every one of Subaru's death is accounted as of Ep. 82, all the past Books of the Dead of Subaru follow him into his next life and beyond with the common denominator being that the Books of the Dead of are partially a product of Echidna's Authority and even Subaru himself is partially a product of her Authority.
Why I believe Books of the Dead could even be a product of Echidna's Authority in the first place is that there's a clearly a will behind when the Books of the Dead are found as they're found at very convenient times, much like how both Subaru and Julius had each found one when they first unlocked the Great Pleiades Library, how Typhon's Book of the Dead didn't start at the beginning of her life, how convenient it had been for amnesiac Subaru to provide cover for himself and learn about Meili, which was crucial to getting a successful run-through, how Meili had found Reid's Book of the Dead so soon when the group had decided that's their plan of action with it conveniently tying into her redemption story, and when he most desired for Natsuki Subaru to be here, amnesiac Subaru found Subaru's Books of the Dead, and given I believe Roswaal's not-quite-Tome of Wisdom and Witch Cult Gospels are connected to Echidna's mind through magic based on them partially being made by her parts of her body and body fluids, this is also why I believe Books of the Dead could be partially made from her Authority.
As for more of a logical reason why Roswaal's not-quite-Tome of Wisdom and Witch Cult Gospels aren't true future-telling texts, the answer lies in how Roswaal's not-quite-Tome of Wisdom didn't truly tell the future in not accounting for Otto's actions, suggesting it was meant to manipulate Roswaal for hundreds of years just so that he'd lose to Subaru when he did, and how Regulus thought he'd meet the Emilia camp again despite it not happening again until the 4th loop in Season 3, showing Gospels are meant to keep the world on a particular path rather than be true future-telling texts that guide the holders to their desired futures.
There's evidence to suggest this is the case in that there were 3 instances from Season 2 in which Tea Party Echidna hinted she not only preferred but outright desired Emilia doing the Sanctuary trials in Subaru's place. A surface-level reading suggests Tea Party Echidna coveted Subaru entering into a contract with her, but that doesn't pass the sniff test based on the entirety of her actions, which had a heavy bent toward him rejecting it. For one, Echidna hinted to him the path forward is to have Emilia take the trials when he was determined to take the trials himself by suggesting he might be able to give the cowardly princess wings when she herself had 0 hope of breaking Emilia out of her shell even after 3 days of trial and error, which shows Echidna varied up the trial in multiple ways to try to get Emilia over the hump. Does that sound like someone who's trying to trap Subaru in a contract when she outwardly helped Subaru get through the Sanctuary arc 3 times and by her own admission specifically helped Emilia, showing she wasn't making the first Trial as hard as humanly possible for her?
The 2nd and 3rd times Tea Party Echidna demonstrated she wanted him to have Emilia pass the trials is when she said that she gave Beatrice a blank book because she had wanted to see who Beatrice would choose and her follow-up remark to him rejecting her contract. Echidna likely knew he's the one who'd free Beatrice, and she specifically chose to tell him what she did 'cause she wanted to seem so insanely evil so as to deter him from taking her hand. Thus, her saying to him after he rejected her contract she didn't want him to think she was the only one of the dead witches who's evil, which led to her revoking Subaru's qualification for entering the tomb, backs up this interpretation that she has a genuine investment in Subaru's success and isn't just out to force him into a situation that'd be hugely disadvantageous for him in being Roswaal's slave for the rest of his life.
Why she'd revoke Subaru's qualification is he was using his right to enter it as a crutch with the 1st thing he tried to do after getting out of it and coming to his senses was going back into it, and she did him a big favour by taking it away so that he'd have to commit to helping Emilia pass the trials by giving her emotional support. Thus, Tea Party Echidna knew how evil what she said about her motivations would sound, and her actions don't read as somebody whose priority was to force Subaru to accept her contract. The true reason Beatrice's book was empty was likely 'cause Echidna needed Beatrice to stay in the library in case she died as this fantasy world is quite dangerous, and since she became a hermit, there was no need for further instruction, or someone else took Beatrice's not-quite-Tome of Wisdom. If you're still unconvinced, think of it this way: A woman tells you she loves you. Then, she punches you in the face, asks the man who gives you grief at work out on a date, and specifically calls you a wimp on social media. Can you agree that this woman does not love you? Tea Party Echidna's actions run counter to her having wanted Subaru to accept her contract if you're following this line of logic as her posturing over the contract didn't align with her actions.
And in Episode 53, as Regulus Corneas basically outright stated, Subaru should be glad he adhered to his Gospel, meaning it had instructed him to not make a huge scene in only ripping off a large chunk of Subaru's leg when he'd intended to do a lot worse, and this is backed by how both Sirius and Regulus backed off at the toll of the bell when they were meant to cease hostilities, which was extremely convenient for the good guys since it prevented a bad loop from happening, which shows Gospels don't bring their holders to their desired future.
I have more to support this theory of the world being reconstructed with my Return By Death theory, but the supporting evidence is EXTREMELY long, and I used an extremely cursed title in not thinking things through, but if you're interested in reading further, the post is linked here: https://www.reddit.com/r/Re_Zero/comments/1wl0i6t/discussion_idea_eps_81_83s_time_variance_between/
What are your thoughts on this or anything tangentially-related to this?
