Epistemological idealism is taking it to the extreme, but yes realistically we are all imperfect beings and any truth we claim could easily be a lie, a mistake, etc.
However Umineko is not preaching epistemological idealism, it’s preaching deciding your own truth. It’s preaching looking at all the facts, clues, etc. But also looking at the heart, looking with love, looking from all angles. It doesn’t give an answer not because an answer is futile, but because we as supposed to find an answer ourselves. The story presents to us Ange’s answer, the answer that allows her move on with her life. But that doesn’t mean we are not supposed to still search for our own.
How much do we know of the other characters? How much do we really know about Rudolph’s shady business ventures and the trouble he is in? About Krauss’s investments and failures? About the struggles of Rosa’s small fashion business? About Hideyoshi’s food business? The answer is very little. But we fill in the blanks with our imaginations, with our own experiences about what those things might be like. The same holds true for SSVD and Eisene Jungfrau. We know they are a form of justice system and we are given some background on them. They are bureaucratic and have paperwork (which Dlanor detests). There is a “great court of heaven”. We see the SSVD arresting a criminal and interrogating them like the police do. We may not know all the inner workings of these agencies but we know enough to make assumptions about what they are like based on our own experiences with similar agencies. This is exactly the same as the level of detail we have on the adult human characters.
Backstory is the cheapest way to add character depth. We get more depth out of Dlanor in her first two scenes (Kinzo’s room and the tea party in the golden land) than many characters get in entire stories. We learn Dlanor’s philosophies on life, her competitive nature, her respect for others regardless of whether they are witches. A character doesn’t need a complicated past to be interesting. A character’s words and actions within a story are always more important than whatever backstory they have. The backstory is what made them as they are, but for the sake of the story, how they are and how they change within it are far more important.
I find that the best way to look at Umineko is all of the ways at once. The meta characters are both representations of ideas, objects, motivations, and people on the gameboard and their own characters with their own thoughts, feelings, motives, pasts, etc. That’s what makes them so special. Many of the fantasy characters work on 4 or 5 levels of the story. Take Chiester 410 for example. She is a character, she was once in love and her lover died and now she finds it easier to simply not love anymore. On the level of the board game she is a gun, one of the 4 guns on the island, used to commit the murders. However to Beatrice/Yasu she is something more. She is one of Maria’s friends, one of the rabbit band, a member of Mariage Sorciere. Her character design and role in the story incoperates all of these elements and forms it around a complete character. Certainly she is more of a side character than someone like Bernkastel, but she is a complete and complex character nonetheless.
That is again because you were not viewing her as a character. Certainly learning all the things she went through enhances the feelings about her suffering as a lifeless being in episode 5, but it shouldn’t be required. There is still the Beatrice that was a character in episodes 1-4. The things she said, the things she did, the ways she influenced the other characters and the events of the story. Her motivations were not clear, but they were hinted at.
Do you not appreciate any character until it has backstory? Do you watch anime and not like anyone until the episode where their past is revealed? It seems like a foolish way of consuming media to me to not invest in a character until you know their whole personal history. Do you meet a new acquaintance and refuse to become friends until you know their life story? Backstories enhance a character, but they don’t make a character. And the best characters are the ones that don’t need a backstory, the ones that everything relevant to the story for them happens within the story itself.