Well, concerning parachutes, the lowest recorded height was at 250 ft by a German Paratrooper WW2 over Crete.
US was at 175 ft from a C-47 dropping troops July 3,1944, this height was due to an altimeter failure and troops should have been dropped at 250 ft.
Both of these instances are with line drop, where the troops jump out and the chute opens automatically being deployed by a line attached to the aircraft.
But none of this really helps you with your specific question of what the lowest height could be. However, if the parachute was an AN-6513, which it probably was, there is a manual on sale at eBay and reading through it the rate of descent feet per second is listed as 14-20 feet per second. A mathematical calculation of chute deploy time and impact( if witnessed) would tell you the height jumped from.
http://www.ebay.com/itm/OPERATION-AN...-/201714156493
As far as the debris field if you have a drawing you may be able to determine aerial break up or large ground impact by size and direction of debris field.
If you have a copy and could send it to my PM I could look at it.
KR
Vince...