Can you help me make a better Spy?

My party recently had a session where, unknown to them, killed a somewhat recognised person in my campaign setting. This was always the plan and when one of the players realised they had a connection to the family of the person they killed (they were the father and master of the house. When their character discovered this, they wanted to be the one to inform this family of their father's death.

My big twist was that when they arrived at this family home, they discovered that the father they had supposedly killed was apparently in their bed resting after a long day at work. The person upstairs resting was to be a Changeling Spy.

The master of this house, the person they had killed had stolen an important artefact that he was supposed to give to the thieves' guild to obtain safe passage out of the city with his family, as unknown to the family, they were going to be in a lot of trouble. The artefact caused the father to transform into a monster which is why the players killed him to begin with. Now the players have the artefact.

The Thieves Guild want this artefact, so they had their resident Changeling take on the appearance of the dead father, used Speak with the Dead to learn more about the father, and has now been lying in bed using detect thoughts on the family, and the identify spell to learn as much information as possible.

Now, my wife isn't the most confident of DND players, she's new to fantasy altogether for the most part, so I was proud that she had recalled information about Changeling and Doppelgangers as a player, but her character wouldn't likely have known what a doppelganger or changeling was. Either way, I didn't tell her to rescind what she said and just continued the game.

Her character then intimidated one of the family staff, a Warforged Butler, to go and check on the supposedly dead person and make sure they were okay and to confirm if the master is who they say they are.

The Warforged Butler said in the moment of returning that it is the master of the house, and that there is no need for concern, Another resident, the daughter, suggested that maybe the person they thought was their father, the one they killed, was the imposter all along, as it must be her father upstairs and backed up the butler, saying that the Warfroged would know if it was someone disguising themselves.

My last thoughts on this are:

• I have no mechanic for the butler to identify the Changeling, but no one Insight Checked him when he said this.
• If he was to have a mechanic, such as Detect Thoughts to try and peer into the mind to confirm who it is, how could the changeling counter this without outright deflecting it?
• When they killed the father originally, the body transformed back form the monster form to his normal elvish form (not a form of a changeling)
• Maybe The butler is either somehow controlled by (or working with) the Thieves Guild now, or he has too much pride to consider he may be wrong.
• I considered maybe there could have been a form of possession on the dead body by the Thieves Guild, but there isn't really a mechanic for this in 5E that would work this way

I guess my question is, do I continue to play as I am, assume that the Butler was just to proud to consider he may have been duped, and reveal that my wife was correct and it was a changeling, or is there something I can do that would make it a now less obvious reveal? Any ideas would be greatly appreciated!