It's actually subtly hinted at when you first enter the Casino. The gang makes a point of saying how the jail looks the exact same as it does in the real world, only the courthouse is distorted.

So for Sae, the jail would have looked the exact same as she assumed it to. Guards posted, inmates present, Akira in his interrogation room, and Goro heading to said room.

That's why the Thieves had to sneak back into the Casino and KO/hide the Cognitive Akechi. Without him around, there was no way Akechi would realize he was killing a fake until the trap was sprung.

What he said.

Basically, Akechi was in the Metaverse without realizing, and he THOUGHT he was killing Joker, so that's what he saw. Cognition is confusing.

To be honest it's a bit bs and only works cos lolplot. I still dunno why that shadow guard took a few minutes to die instead of most shadows that die instantly, and they had no way to test that BUT PLOT. There's loads of other small issues like that too.

They do kinda lay the brickwork for it as early as Kamoshida's Palace.

But yeah Akechi not catching on required a TON of luck and would have been impossible had Akechi suggested any target other than Sae.

But if saw was also in the meta verse, how come she didn't understand morgana?

.... I have to imagine that's the translation team's fault.

Because it seems at the least you need to HEAR Morgana, at worst you need a Persona yourself. Though the dialogue kept saying you just had to go to the Metaverse.

Posting a lengthy reply (trying to find character limits)

Basically, here's the deal:

When they fond out Sae was going to be the investigator, they used this knowledge to get into the interrogation by going into Sae's palace, and walking there.
Sae never lost her treasure, she had a change of heart on her own will, which was part of the plan. They needed her palace in tack for this to work.

So, I have this entire post written up, and the site is refusing to let me post.

Give me a minute to figure out why.

https://pastebin.com/RcsaJFEc

Here's a quick Pastebin.
This should help clear something up. Ask me for clarification on anything.

This shit still astounds me , even in writing