To (hopefully) answer some of the points made above...
My understanding is that when wartime aircrew failed to return, their possessions were collected and forwarded to a central depository, in the hope that they would return - either through escape or evasion, or as released POWs - and resume their flying. In due course, personal posessions would be returned, but logbooks seem to have been retained longer. As Simon K, points out some logbooks were eventially claimed and passed to next of kin.
Chug's calculation of the number of logbooks makes general sense, although some of the more experienced aircrew may have had more than one logbook, and the thickness of logbooks varied - the fact that later wartime ones had fewer pages is partly due to the shortage of paper, but sadly also suggests that they realised some would never be filled...
Logbooks are official documents, but not all official documents make it to the National Archives. I can only assume that the 6400 ft figure was considered too much to transfer in its entirety, and a sampling exercise would privide sufficient evvidence of the sort of things that aircrew got up to. As we know, however, every logbook is different and potentially valuable for research.
ORBs were typed in (I think) triplicate. The top copy should have gone via the Air Historical Branch to Kew, but I've seen some rather dire carbon copies, and some ORBs seem never to have made it at all. Some have remained with the units that raised them, but they are at the mercy of those who have little regard for history.
Finally, Searchlight Cooperation seems to have been more about acting as a target than being on hand to attack targets illuminated by them.