This combines a lot of popular theories in these threads, but I want to give a go at one unifying theory in a tl;dr post. I should probably separate these sentences into red and blue truths but that’s like citing a legal memo and I do enough of that already and I’m lazy.

In reality, the apocalypse happens. Whatever humans survived are put into the human processing plants we see in Ciconia, and they become brains in jars. All the brains in jars are hooked up to a huge computer that can put them in whatever Matrix scenario it can come up with in the form of ‘pieces’ each controlled by a single brain. A brain is only supposed to control one piece per gameboard, but some brains might be able to handle multiple pieces across multiple gameboards. This is why Okonogi appears in every gameboard without being a voyager or territory lord. Let’s call the program that creates scenarios ‘gameboard.exe’ and let’s call the AI running everything ‘Featherine’.

So like Ciconia describes, all the brains are totally sensory deprived before they’re hooked up to the computer, then they’re linked together. As of this week, we learned of an entity named ‘Anomalous Spinal Specimen LD3105’. Why is this specimen anomalous? Because it never properly connected with the computer. So the brain just sat there in the jar, totally without senses, connected to the computer but unable to interface with it. This becomes LD’s greatest fear, the nothingness that her eventual incarnation, Lambdadelta, will express when discussing the logic error with Battler.

Somehow, some part of the link reaches LD. At this moment, young Takano is the piece of a connected brain existing in the Hinamizawa simulation. LD links with Takano at the moment when the lightning bolt is about to strike her. LD is unable to manifest as a piece in the Hinamizawa simulation, but she can use her existence as ‘something that should not exist’ to alter the simulation so that the bolt misses Takano. LD attaches herself to Takano, bestowing upon her the absolute will that compels her to kill Rika with absolute certainty in all incarnations of Hinamizawa. This is not LD’s objective immediately, though, and she lies dormant within Takano and suffers through Takano’s life along with her.

As infinite incarnations of gameboard.exe run and reset, Featherine observes and records all outcomes. Eventually, she gathers such a massive quantity of data that she risks being unable to function if she continues to collect it. Therefore, she needs to localize her recordings of specific gameboards within the gameboards themselves. These are basically like additional hard drives and they essentially function as Featherine’s ‘memory devices’. When Featherine has an avatar, the memory devices are represented in the form of her horns. Only people with a certain awareness of the true nature of reality, or those that are connected to Featherine, can her horns.

The memory device in the Hinamizawa gameboard is the Oyashiro statue. One day, a young Satoko breaks the hand of the statue, therefore breaking the memory device. This causes Featherine to glitch out whenever she tries to interface with the gameboard, resulting in dumb god Hanyuu. Now, this would normally not be a major problem; Featherine wouldn’t place her memory devices within gameboard programs without a way to repair them, right?

LD, being aware of her own true nature because of her extended isolation and imperfect connection with the gameboard, would sense this memory device instinctively. This is why Takano is drawn to the Saiguden and displays genuine enthusiasm about it in multiple loops. So when she learns that the memory device is damaged, she knows it is her time to strike. Again, because LD is aware of the brain jar computer thing because of her unique isolation circumstances, her goal is to meet Featherine and, ideally, supplant her.

Because Rika has to have some kind of connection with Featherine, maybe she’s the one that is designed to interface with Featherine should the situation require it, killing Rika would essentially sever Featherine from the gameboard program given the damaged statue. Featherine has thought of this, however, and has a failsafe to reset the gameboard should Rika die while the memory device is damaged. This failsafe is the Great Hinamizawa Disaster which resets the simulation from a certain point influneced by Hanyuu but, unfortunately, Takano is tasked with administering. This causes all of Rika’s loops in the Higurashi Eps, but her piece can remember most of everything due to her connection to Hanyuu.

So Takano’s absolute will kills Rika with certainty for 100 years until the miracle kakera happens. LD compels Takano to do this in hopes of summoning Featherine, but unbeknownst to her, Featherine has been summoned as Hanyuu, so meeting her won’t do much good. But anyway, Rika wins despite the whole gameboard program being rigged against her through LD’s influence. This allows the Hinamizawa simulation to continue to the point where the statue can be fixed, and Hanyuu’s memory and Featherine’s control of the gameboard can be re-established.

Takano has essentially exhausted her usefulness as a piece at this point, so LD abandons her and attaches herself to the piece closest to Rika: Satoko. This causes Takano to break down whenever she is supposed to kill Rika, because her absolute will has vanished. LD compels Satoko back to the Saiguden where she interfaces with the now repaired statue, allowing her to meet Featherine. At this point, LD has grown strong enough to influence the Ciconia gameboard but not the Umineko gameboard, which is why Featherine recognizes Vier as her piece. Expanding on that: Takano and Vier are pieces controlled by the same brain, like Okonogi. So when LD influences Takano, this influence seeps into Vier because of their shared brain. Satoko has no idea LD is trying to hijack her role as a piece from whatever brain is controlling her, so Featherine’s words sound like gibberish. Featherine, recognizing this as a highly amusing and unforeseen scenario, gives LD the territory lord powers with Satoko as her piece that she never had when she was attached to Takano. This is basically equivalent to how Bern’s piece, Erika, briefly became territory lord of the Umineko board.

So Takano’s absolute will to kill Rika, carried over through LD, basically passes on to Satoko when she becomes territory lord, merging with Satoko’s close bond with Rika to quickly form the homicidal, delusional Satoko who drops the chandelier onto Rika after only a single loop. LD’s goal is now to use the Satoko piece to go from territory lord to voyager, so she can insert a piece into a gameboard other than Hinamizawa, or grand a piece in another gameboard territory lord powers.I’m having a hard time establishing the mechanics of how this would work, but something like LD’s anomalous nature combined with Satoko’s territory lord status add up to make a voyager if she completely masters the Hinamizawa board. Rika’s submission would be the win condition that upgrades her to voyager.

Whatever happens in the last 3 episodes of Gou, Rika and Satoko both become the voyagers Bernkastel and Lambdadelta. Rika will accomplish this somehow because of her connection with Featherine. TBH I think Rika will voluntarily become a voyager, to the delight of Featherine, to pursue and stop Satoko, facing her ultimate fear of and endless nightmare existence all for the sake of her closest and dearest friend. But anything could happen.

Lambda can now directly influence pieces and give them endless abilities across all gameboards, This causes her to create Beatrice and therefore all of Umineko happens. Bern pursues her and, alternately, makes Ange and Erika her pieces. Bern pulls a spectacular heel turn to become the main villain of Umineko all without breaking kayfabe once, all to protect the gameboard from Lambda. Why is the Umineko gameboard so important? Because of the brain controlling Maria’s piece, of course. Maria is the Witch of Creation, the ultimate anomaly in the computer who can take zero and make it one. In other words, she can create her own gameboards without the help of Featherine! Whoever controls Maria, controls the universe, so I’m pretty sure she’ll come back later and be super important in the grand finale of WTC. Anyway, Bern, with a Snape or Revolver Ocelot-esq motivation, is attempting to engineer events with her own agenda in the war between Featherine and LD, all out of love for Satoko! She masquerades as Featherine’s loyal miko, biding her time before she makes her move.

So Umineko happens and Lambda gets to meet Featherine, but Lambda dies lol, so that’s a setback for her plans to expose and/or replace and/or get revenge on Featherine. It matters not, for now she can insert her true piece into the Ciconia gameboard: Jestress. Working with Vier in a pocket of the simulation away from Featherine’s notice, probably under the glass sea, Lambda prepares to unleash her trump card: Miyao. Just as LD was a brain influencing multiple pieces, Miyao has been specifically engineered to be the opposite: a single piece influenced by a whole network of brains. I guess that would be similar to LD and Satoko comprising Lambda in a lich king kind of way, so maybe the preceding sentence is wrong. Anyway, this is why he has Meow, blue man group Miyao, and who the hell knows what else in his mind. This essentially makes him the antichrist of the WTC universe, and he is probably being guided by the conspiracy to a grand confrontation with Featherine (God) in order to crash the whole system once the conditions of the Ciconia gameboard are optimal after multiple (read: 4) loops. Bern piece is either Koshka or will be introduced in the next Ciconia EP if that ever comes out.

There’s all this Biblical and gnostic shit, eg Featherine is the demiurge, four Ciconia factions are the four horsemen, etc. that I could talk about but I need to actually re read Ciconia to actually get a full grasp of the religious shit because it’s all in that series. And there's the whole thing about how the L in L5 doesn't stand for 'level', but for 'lumbar' and how Hinamizawa Syndrome is simulation awareness but that's a whole other textwall. But bottom line that’s all of WTC solved by yours truly and also Battler and Yasu brains still exist somewhere so the nanomachines could make them bodies for their brains and they can finally be together and be happy for real so that’s pretty cool.

Edit: It occurred to me as I was editing this that my interpretation of LD3105 is almost exactly like Lambda from Tales of Graces. Was not intentional even though they're both called Lambda! But now that it occurred to me, this is actually a perfect illustration of what I'm getting at. In Tales of Graces, Lambda is this weird extradimensional entity that can only manifest fully when attached to a host. First, the scientists made Lambda an android body, but the plot happens and Lambda has to attach itself to Prince Richard. It lies dormant within Richard for years until again, the plot activates Lambda who then proceeds to steer Richard's actions towards its own goals before possessing him completely. To stop Lambda, protagonist Asbel has to willingly allow it to attach itself to him. So basically, LD is Lambda, Takano is the android, Satoko is Richard, and Rika is Asbel. Asbel allowing Lambda to attach to him would basically be the equivalent of Rika beocming a voyager to stop Satoko. Notably, there's a big emphasis in that game on nonviolent resolution, which R07 is obviously a big fan of. At the climax, Asbel empathizes and talks with Lambda instead of attacking it, which turns out to be the key to pacifying it. We already know R07 is a big JRPG guy, so maybe he played this one!