Without having examined the patient, I would start from the simplest diagnosis, which is that the patient's... identity retreated or fled from the predator.
The observed lack of interest in things that were of interest prior to the possession, and the fear that the predator may return, both point to this.
That and soul-casing is finite, and -- for adults -- fixed. If something invades, something else will have to retreat, or the soul-casing will strain or fracture, which...
Which can be looked at after ruling out a simple evacuation. If it has fled, it can't have gone far, and should still maintain a physical attachment to the patient.
Has this hypothetical patient undergone any hypothetical treatment?