An argument like "but Battler as a culprit renders 95% of Umineko meaningless and deceiving mess" is not valid because as of Ep8 95% of Umineko is confirmed to be just that because it turns out
95% of Umineko was fiction written by different parties with vested and conflicting interests in the (mis-re)presentation of events on R-prime.
Now imagine the following scenario. We have a "black box" type of incident with 3 survivors: Battler, Eva and (probably) Yasu (whoever the hell that may be, let's assume it's a servant person introduced in Ep7).
They all write their (biased) accounts of the events that transpired on R-prime. In a sense the whole of Umineko should be considered one huge epistolary novel (novel in letters, that is).
The fictions (letters) are as follows:
1) Legend (message bottle 1) - written by Yasu
prior to the incident; purpose - it is a script for her fake death spectacle and a homage to a certain Christie novel;
2) Turn (message bottle 2) - written by Yasu
immediately after the incident, purpose - to cover for Battler as the real culprit at the same time providing some hints to the real identity of the culprit for the most astute reader; that's why it's impossible for any outside party to get the message; that's why Turn is so complicated and devoid of clues; it's intended recipient is Battler and him alone; may be interpreted as a kind of "Why, Battler, why?" type of message; the reason Turn has to be written after the incident is because of Will's final battle with Yasu in Ep7 - for the whole Legend episode Will uses "illusion to illusion", but in Turn the "illusion" part gives way to "earth to earth" (=real murders, real corpses) starting with the re-killings of Nanjo, Kumasawa, Gohda, George and, most importantly, Shannon, i.e. this is the part which first suggests the highjacking of the fake murder game by the opportunity killer being, of course, Battler, his parents or any combination thereof; but the funny thing is, only Battler survives which makes him very suspicious in my books; the knowledge of the highjacking can only be possessed by the person who: a) set up the game in the first place; b) has a good idea of who the actual culprits were and c) survived the incident to tell he story.
Speaking of Turn, I can completely explain all of it's locked room tricks with anyone from the fake 1st twilight as the culprit (as far as anything can be taken as explained without the confirmation from the author of the work itself). But what makes the least amount of sense in Turn are not the tricks themselves, but the existence and authorship of the manuscript of Turn in the first place. If we posit Yasu wrote it, she could have done it
after the event only. It's either that, or R07 is a hack writer with no respect for his readers at all.
3-4) Banquet and Alliance - probably both written by Yasu after she had time to settle down, so to speak. Here her intention shifts from passively shielding Battler by non-disclosure of facts to actively scapegoating someone else (and who would make a better scapegoat than the only officially confirmed survivor, Eva?)
5) End - probably written by Yashiro Ikuko alone while Battler was struggling with his amnesia (on a side note, R07, we really deserved better than that; amnesia, seriously?);
6) Dawn - written by Battler; the most meta of all of them, only clue pertaining to events on R-prime is the reinforcement of the idea of "re-killing the fakers"; otherwise, it's all Beato-heart this, love duel that; on my first reading I was mighty pissed with that, but now it's all coming into focus: if Battler is the culprit, it only makes sense his novel would be all about trying to revive and understand Beato, because as the culprit of R-Prime he has no need to uncover the truth of the incident, because he knows it already and it's to his detriment anyway, so he focuses his attention on the only part of it he could not understand all that time (namely, the idea that a retarded 13 year-old could fall for him when he was just teasing her with RABU and come up with the plan he ended up (reluctantly?) using for his own ??? PROFIT). That's why Battler ends Dawn with that nonsensical "lived happily ever after” ending. It's his way of atoning to Yasu, it's like he's saying: "Yep, sorry, I used you all the way and now you're dead and forgotten while I'm alive and well to do, but let's at least give you the proper send-off, if only in a piece of trash fiction";
7) Requiem - probably written by Yashiro Ikuko alone for reasons unknown (maybe the series were getting an rabid Internet fandom at the time so it was, like, for commercial reasons only
) Mind you, it answers nothing so it was probably safe to publish.
8) Twilight - first half of it shows Battler doing everything in his power to avoid the truth of R-prime uncovered with happy BS ala Dawn's ending; the latter part provides some vague answers, paints Battler as an unwilling accomplice, frames Rudolf/Kyrie, goes to great length to exonerate Battler of any killings; all of it written by that same Battler of course.
On a side note: who was Okonogi working for all that time? Maybe Ange couldn't be pacified by mere fiction anymore, so Battler decided to shut his annoying sister up for good?
Anyways, I’m all with Will Wright on the subject that “good Ange is dead (preferably horribly mutilated first) Ange”; she really is the most annoying, useless and unnecessary character in the whole of Umineko. And if she is supposed to represent us pro-mystery readers, I’ll go with goat-kun any day instead, thank you.