How I see things:
- To prove that Erika knew everyone was in the parlour she should have heard it. She wasn't there when Lambda told it to Battler. We don't know if she was there the first time the scene was played, if Lambda told it the first time the scene was played and, in case Lambda told it and Erika wasn't there, if Bern informed Erika of this.
- the scene is played as following:
1) Genji knocks at the door, Erika enters in the parlour, Shannon and Kumasawa are following her. In the parlour there should be the siblings (the cousins aren't there yet and the text doesn't mention Nanjo, Gohda or Kanon). Erika is introduced to Krauss. She's likely aware of who's in the parlour and possibly where and likely can feel Shannon and Kumasawa behind her but of course NOT EVERYONE IS ON THE PARLOUR right now.
2)
MetaBattler stops the game and asks for clarification about Erika
Quote:
"Repeat it. ......'Furudo Erika had no influence on any of Beato's games before now'."
"Sure, I'll respond. Furudo Erika had no influence on any of Beato's games before now. She's nothing more than an extra character who appears for the first time this round. She does not exist in the worlds before this one, nor does she influence them."
"In that case, what happens to the number of people on this island right now?"
"Of course, it's plus 1 over the previous number. But don't worry. Furudo Erika only increases it by one person. Besides her, the number of people on this island is exactly the same as it was in the previous games."
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As Bern made the detective proclamation (and likely she made it in the previous time in which this scene was played as well) I think she was aware that Erika couldn't affect the game board so I've no idea if she bothered checking the effect Erika might have on the game board.
3) The cousins jons the others in the parlours and Erika is introduced to them. They exchange some words which likely means Erika is looking at them while talking to them.
4) Meta discussion goes on while Erika is still talking with the cousins
Quote:
"......Isn't that convenient. This way, the number of people is clear. The people in the guesthouse have joined up with the rest, and now, all of the pieces except for Kinzo...all of the humans, have gathered in the parlor."
"Looks like it. In other words, the number of people in this parlor now is equal to the total number of people on this island."
"There were supposedly 18 people on the island, ...but since Grandfather was actually dead, that became 17. ......And now one piece of Bernkastel's has been added......"
The number of humans on this island has returned to 18......
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5) The we've this:
Quote:
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'I' glanced around at the humans in the parlor.
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I'll assume I refers to piece!Battler. What follows is the description of where everyone is:
Quote:
The guest, Furudo Erika. And behind her, Kumasawa-san and Shannon-chan. Off to the side was Genji-san.
Krauss oji-san and Natsuhi oba-san were welcoming the guest.
Gohda-san immediately started showing off, and Kanon-kun was being unsociable, wearing his usual blank expression.
There was Dad and Kyrie-san. Eva oba-san and Hideyoshi oji-san. Rosa oba-san and Maria. And, Doctor Nanjo.
Then, on either side of me, were George-aniki and Jessica...
This is everyone. The true number of people on the island at the moment......
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... while Erika is still talking with the cousins with a dignified and appropriate behaviour.
By this description we have that when the proclamation 'everyone is in the parlour was done' piece!Erika was busy talking with the cousins and looking at them with a dignified behaviour so she couldn't really turn her gaze all around the room. We know for sure that Kumasawa and Shannon were behind her so out of her field of vision. We don't know who else she could see apart from the cousins so more people could have been out of her field of vision.
In short, in the moment that red was said piece!Erika's perspective of the people in the room was limited and remained limited for a certain period.
The narration doesn't tell us when she quit talking to the cousins and was free to look around. It's possible she never managed to do it or that she could do it only after 'the servants left the parlour'.
Checking the number of people on Rokkenjima is important for Battler who hopes to find an extra person. Bern knows there's no such thing and wants to frame one of the Ushiromiya. Ergo Erika might have never cared to check if people were actually less than what the story implied in EP 5 so piece!Erika and meta!Erika never bother to confirm if Erika could see everyone in Ep 5.
The best Erika's photographic memory could tell her is maybe she hadn't seen Kanon entering before the cousins entered or with them, however she's short and Kanon is short as well and George and Battler might have been blocking her view (and as a servant Kanon had to enter after them). For all we know she might not have seen Gohda entering either.
As long as Shannon used the moment in which Erika was busy talking with the cousins to leave, change into Kanon's clothes, show up for a second in order to have Erika catch a glimpse of Kanon, the scene isn't that suspicious... expecially if you've no plans to check how many people there are in the room.
The scene with everyone present in the parlour is described by Battler who's interested in checking if everyone is there so it can include either Shannon or Kanon. If Meta!Erika saw it, it likely was through piece!Battler's eyes in the same way as she likely saw piece!Battler meeting piece!Kinzo.
In short, while everythng in piece!Erika's view was real, everything that wasn't was merely a fantasy scene.
The 'Battler met Kinzo while Erika has her back turned' scene was showed even when the game was played the first time so I guess Erika had again her back turned while Kanon and Shannon were shown together.