Issi noho,
You're fairly close to the truth. The reason you can't do a Cat2 approach followed by a manual landing is due to the size and the position of the valley on the approach to 06L at MAN.
As we all know, when an aircraft descends on the ILS the corrections required (in order for the aircraft to stay on the glide-slope) are also reduced. The Flight Guidance System (FGS) is programmed to take this into account. The input for the increased sensitivity of the ILS is from the Radio Altimeter.
Unfortunately for 06L, the valley occurs at a very inconvenient place, ie. when the aircraft is 100feet above the touchdown point - which would be the normal Cat2 DH. There's a chance the FGS could be making a large correction at this stage as the input from the Rad Alt leads the system to believe that it is higher than it actually is. At the Cat2 DH the aircraft should be stable. The concern is that when the autopilot is disconnected, the aircraft could be chasing the glideslope which could lead to a destabilised approach as the aircraft subsequently deviates from the required profile.
On the Cat3 approach for 06L, the valley isn't an issue as the FGS will continue flying the flying down to the touchdown point.
To summarise then. A Cat2 approach followed by a manual landing can't be done on 06L due to the fact that the pilot could be taking control at a stage when the aircraft could be slightly destabilised.