The Intellect Devourer's Body Thief Action states that the Intellect Devourer "takes control of the target's body." We know that the body of the victim is still alive and counts as a creature.
The Possession of Mind action states, "The mind now controls the body, but does not rob the target of consciousness."
So does the mind override the intellectual devourer to control the humanoid in question?
Also, in terms of charisma salvation, salvation would automatically fail because the ultimate goal is the humanoid, or would the Intellect Devourer try to rescue the rescue with his own charisma of keeping control of the body?