The answer to the original question has been bugging me since I first read it and I can now give a definative answer.
Whilst copying the 38 group RAF Operations Order for Operation Comet - The planned attack on Arnhem cancelled a week before Operation Market Garden, I came across the order in para 17 (i) To the effect that tug aircraft were to drop the tow ropes 2 minutes after releasing the gliders, they were then to turn and follow a reciprocal course home.
I've no doubt that this was standard procedure and as such little or no thought was given to what was under the aircraft when the tow cables were released other than to avoid gliders on or over the LZ.