Schnaufer did not tend to 'hang around' to watch the result of his attacks. The Lancaster obviously suffered catastrophic damage ( the nose breaking off, and no survivors ) so it was reasonable for the Schnaufer crew to assume that the aircraft would have gone into the sea which is quite close to where the interception took place.Also it would not have been feasible for Peter Hinchliffe, when writing Schnaufer's biography, to minutely examine every the circumstances of every claim.
So - it really does at last look as though my little mystery is solved, and that the pile of wreckage lying in a hangar at Wickenby airfield represents one victory for the most successful nightfighter pilot of all time.
( But you know what it's like with research - I'm waiting for someone to contradict me....
)