dnd 5th – NPC1 wants to play NPC2 for the murder of NPC3, without my players having an easy way to find out

I do not just want to do the DC 50, it seems cheap.
I do not want to use the simulacrum for the same reason.

I would like it to be something that is not beyond the reach of the players.

Here's what I've

Both houses are protected against surveillance (NPC2 and NPC3 are of political importance).

Use the magical aura of Nystul to prevent groups of spectators from seeing / detecting the disguising self.

Disguises himself as NPC2 to disguise himself as disguised as NPC2. He then goes from the house of NPC2 to that of NPC3. He is seen by a few people, one of whom sees him coming in with a bowl of lids, without realizing the observers.

The victim is found dead from a powerful poison the next day. The investigation continues with a feather analysis. Witness reports place him on the scene, carrying the bowl containing the poison.

He is quickly arrested, the party discovers heralds (they are currently outside the region.), The party was a friend of NPC2 and went in search of the truth.

I plan to reveal this deception to the party later, in a controlled mansion, by going on the hunt for evidence.

What are the holes in this plane? What spell / rule / etc could make that all fall apart like a house of cards and do you know a better way to run it?

Thank you.