I don't recall it ever being said -at any point- that there had been "sighting of the figure in the upstairs bedroom, alive and with mobility." My understanding has always been that it was a "trick of the light".
It was 'not a trick of light', it was the sighting of Ralph Neville, moving around inside the bedroom. He 'was' Sheila's accomplice. He helped her to kill the others, including overpowering Mr Bamber, Snr...
Neville used 'Anthony Pargeters' .22 bolt action (Bruno) Rifle in the shootings, and his Parker hale silencer. I am correct in reporting that Neville may have been injured during the struggle with dad in the kitchen, and that 'his blood' might be the source of the blood attributed to the silencer (A, EAP BA, HP 2-1, AK1)...