[jjblue1]

Okay... since I think among us there are more than one theories about why Erika didn't see Kanon here's mine.

This is my theory/understanding about how the gameboard work.
Note that Umineko never states clearly how it works so I guess it becomes a catbox open to more than one interpretation.
If you've different ones I'll be happy to hear them.

Oh and the following spoilercut are merely so that this post won't be too long.
Although they contain references to the games they're mostly a list of theories.

Spoiler for How the gameboard work:

1) Pieces are just pieces. Meaning a piece has no more freedom of action than your ordinary fanfiction character or your roleplay character, and if there's not someone 'playing' with them they're left there to take dust. However rules for moving the pieces is they have to be more or less in character unless there's an explanation for it (Ex: George can't suddently start wearing female clothes and talking like Jessica, unless there's an explanation to it, for example they're playing some sort of game) and that they've to keep their own abilities (George can't forget his martial arts, nor Kumasawa can know them in his place unless there's an explanation for it, for example George is faking not knowing them and Kumasawa using them is just a fantasy scene). This also refers to knowledge (Battler can't know all of sudden things about magic circles while Maria suddently forgets them).

2) The human side can move 1 piece at time (Bern didn't move just Erika, she also moved Battler and likely she moved alternatively Battler and Erika during the epitaph solving), the Gamemaster (GM) can move more than one pieces at the same time (Pretty obvious as the GM controls what everyone does. Lambda also moved Battler to oppose him to Erika in the fight in Kinzo's room. If Battler was solely on Bern control or she could have moved Erika and Battler at the same time, Lambda couldn't have used him to oppose to Erika's reasoning).

3) Pieces has also no more consistency than your ordinary fanfiction/roleplay character. Meaning they can't see or touch anything.
However, to make things clearer for the player, he's given a visual of the scene... though the narrator's eyes. However, as the narrator isn't reliable unless he's also the detective, the visual of the scene can trick the player.
Anyway, if you assume pieces are placed on the equivalent of a chessboard, you will have to place them according to what the narrator is saying. The rule for placing them is that fantasy pieces (Gaap, living!Kinzo, Ronove, Siestas, Purgatory Sisters and whatever constitutes magic like butterflies, magic towers and so on) can't be seen by the detective, so the narrator must place them out of the detective's sight. As Shannon and Kanon can't be seen together, it's impossible to place them both in the detective's view. However, as long as it exists a dark spot in the detective's view, they can be placed in the same room.

4) Pieces are 'moved' through oral description of their movements (though likely there are records of the game as a player can go through it again). Description can be detailed or vague. The player must declare his moves to the GM. Interpretation of the description of the movements however can be tricky (Erika can say she 'will try to discover the dead or alive status of a person' and mean she 'will kill said person'. Battler says he'll 'hand Erika enough tape to seal 3 rooms' and Erika 'will be able to use it on everything in the room to her heart content and as much as she wants as long as it's used only to seal 3 rooms completely'). This allows the player to 'move' a piece in ways 'unknown' to the GM, where 'unknown' means merely the GM might not figure out which interpretation the player wanted to give to his/her words.

5) Interpretation of the pieces' actions can be changed through the game as long as it isn't stated in red (Once it's said in red 'I proclaim that the chain lock has been repaired by the duct tape seal and has regained its original functionality. And, thanks to that, I locked the room upon entering it and made this guest room a closed room from the inside once more.' the fact she 'repaired the chain with duck tape' can't be taken back and the same applies for 'she completely severed the heads of all those she killed.'...) though as red also can be subjected to interpretation you can try and force your way out of red.

6) As pieces aren't real people they can technically move in ways that defy realism and look more like a 'miracle'... as long as isn't absolutely impossible for them to do so (for example they can't fly but they can move in an extremely fast way).

7) Certain pieces can use special moves (es: detective authority) though due to this they might lose the chance to use other moves (ex: if you use detective authority you can't be the culprit)

The player is either supposed to know or guess the overmentioned rules... though he can be also told about them if he/she has a witch on his/her side who's feeling generous.

How, according to this, the detective's reliable perspective work.

Spoiler for Detective's reliable perspective:

1) the player can see a representation of what the narrator is 'seeing'.

2) the GM must make sure that, in the representation of the scene, magic scenes are out of the detective's field of vision and, of course, he must have a 'logic' explanation for them. People saw a table floating behind Erika? Either is a trick or they're lying as floating tables don't exist.

3) the player can't see what his piece is seeing as the piece is nothing more than a mental creation that's basically seeing nothing. If his piece is also the narrator though, the player can see a representation of the scene through is piece's eyes. If his piece is the narrator and the detective the representation will be thrustworthy.

4) how reliable perspective is useful if actually the player is seeing nothing from the piece's perspective?

- If the solution of the mystery of the 'floating table' is a lie, the detective can't say he/she see the table floating. Either the table stops floating as soon as the player says the detective will turn (ES: Battler sees Kinzo but, as soon as Erika turns Kinzo has disappeared) or the detective is stopped from turning/seeing what's behind by something (ES: Battler could have realized Shannon's body wasn't in the garden shed but the adults did his best to stop everyone from entering. In EP 5 Erika could have realized the cousins weren't dead is she had looked. The GM stopped her from seeing this covering the 'bodies' with covers and she also tried to have Battler stop her from entering). Of course a lie had better sound realistic and the same applies to the way the detective is stopped from checking or it will sound suspicious (ES: in EP 5 there was a battle of red and blue over how the letter was placed because a letter appearing out of nowhere was obviously suspicious).

- If the solution of the mystery is a trick clues must be present. Erika's abilities will cause her to unfailingly 'spot' the clues or better, 'if the player has Erika search for them the GM must hand them to her'... (In EP 6 Erika is sure no one is hiding Battler's bathroom because 'she had it confirmed with the red', not because she had checked everywhere, and the same applies to the room Battler was supposed to be in) provided that piece!Erika will search for them and that she will search for them in the right spot (ES: A piece sees a table floating in Kinzo's room and piece!Erika goes to search for clues in the kitchen... when they're hidden in Kinzo's room. The GM can safely say Erika found no clues in the kitchen).

Either way having the narrator showing something clearly magical when the detective is around can make easier for the player to spot the trick used... or, in Umineko's words, 'the detective's magic-resisting toxin might destroy the magic'.

Scenes where Shannon and Kanon are in the same place as Erika and how everything I theorized applies to them.

Spoiler for Scenes where Shannon and Kanon:

- The parlour one. They're supposed to be there while she's talking with the cousins. As Shannon is placed behind her, she's surely out of her field of vision. It's not sure if Kanon is as well, though since 'everyone is in the parlour', one of the two must be there. As in that moment they can't both be observed by Erika, nor she tries to observe them both at the same moment, the illusion of them existing at the same time is safe.

- Out and in Kinzo's room. They're supposed to be there while Erika is investigating. However Erika is focused on Natsuhi from the moment she said she was the last to see Kinzo. They can be placed behind the siblings and the other servants and, as everyone apart from Erika consider them as accomplices, no one will mention there's just one of them and will do his/her best to hide them from view.

- Erika questioning people. It's never confirmed if Erika tried to question them together. If the player declared 'I'm going to have the Erika questions the pieces one after the other' Shannon and Kanon didn't need to show up at the same time. Also, at a certain point, Natsuhi was drinking tea. Since someone must have done the tea, it's a hint everyone wasn't present when Erika was during the questioning (and technically in the scene Shannon, Kanon and Kumasawa are never showed).

- 'Hideyoshi's room'. They were supposed to be present when the corpse of Hideyoshi was found. Erika joined later so the first part of the discussion can be a fantasy. Note that white text, speaking of how Eva returned with help, says 'Eva dashed back, bringing everyone with her.' However the white text mentioning 'everyone' is clearly lying as Natsuhi is in the closet and Erika is checking the shutter.
Erika joins just when Battler is saying the trick to the closed room might be the shutters can be opened from outside or that one can escape from the bathroom. Eva was probably already arguing with Nanjo about moving Hideyoshi's corpse. All stuffs that can distract a player from checking if 'everyone' is really in the room. And again is possible to place Shannon and Kanon behind the adults' backs... though they can't leave until Erika moves away from the door. At this point Battler demands again that Erika will move out of the way, and Erika will move away further, though his action might have meant to cover her view. Now that the people were leaving the room and that, in order to do so they've to pass close to Erika, it was starting to become possible to notice that only Shannon was present (Shannon went to retrieve the covers with Kumasawa so likely was more visible). If however Battler places in front of Erika he's tall enough to stop her from viewing clearly the scene. At this point it can be assumed that 'Kanon, who Erika never spotted because he was still behind the adults, left with them'. Once out Kanon and Shannon both speak. Erika hears them but Kyrie immediately talks to her, likely stopping her from leaving the room and seeing that out of it there's just Shannon. Note that Erika also let herself be stopped from checking the closet.

- the parlour again. It's Kirye that declares that they have "decided to stick together from now on. First, we're carrying Hideyoshi-san's corpse to the parlor. Can you hold off playing detective until we get there?" implying they're all in the parlour though once Erika joins with Natsuhi she uses detective authority to say none of the pieces have a right to escape the discussion.
The scene is tricky because Erika says "... Oh, sorry. Could you close that door for me?" and we're shown both Shannon and Kanon. The answer is "Y, yes..." but we're not told who said it only that they closed the door, thought it's Kanon that locks it.
However it is possible to say nowhere is said that Erika looked at them while saying that sentence. This makes possible to work a scene in which Erika was still staring at Natsuhi while saying out loud that sentence secure one of the servants would move to close the door.

Note that to use Umineko's words all the overmentioned solutions are:

... an unfair trick......
If it was a sleight of hand, it'd just be a cheap one.
If it was a mystery, ...it'd just be cheating......
However, in this witch's game, it's a legitimate......'logic trick'...

So the solutions doesn't fit a mystery, merely a logic trick.
I guess I'm not good enough to find a solution for a mystery, just for a logic trick so, if you've a theory for a mystery solution, please share it.
Note that if there's no solution fitting for a mystery, even through finding the solution for the logic trick was fun, I'm disappointed as I prefer mystery to logic tricks.