5th dnd – Can disguise of self can be used to condemn an ​​NPC for a murder without the knowledge of the players?

I find myself in a situation where an NPC (Adam) wants to indict another NPC (Bob) for the murder of a third NPC (Chris). I try to figure out how to get there without the players being about to find out easily.

I could give a check to players with a DC50 but 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

  1. Bob and Chris are politically important. As a result, their homes are protected from surveillance.
  2. Adam will use the magical Aura of Nystul to prevent the parties who are watching him from seeing / detecting him.
  3. Adam uses self-disguise on a loyal third (Dave), disguising him as Bob
  4. Dave then moves from Bob's house to Chris's house, seen by a few people.
  5. One of the witnesses sees Dave come into Bob's house with a bowl with a lid
  6. Dave will obviously not notice the witnesses
  7. Dave kills Chris with a potent poison from the bowl
  8. Chris is found dead the next day
  9. An investigation by witnesses and witness reports brought Bob to the scene, carrying the bowl containing poison.
  10. Bob is arrested quickly
  11. The party learns of Chris's death and the arrest of Bob by heralds (they are currently out of the region)
  12. The party was friends with Bob and went in search of the truth.

I intend to reveal this deception to the party later, in a controlled manner, by starting to look 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?