Dougie,
a very nice account of an HSP drop. Time certainly was relative as Einstein said. When I first started dropping everything seemed such a rush. By the time I finished I seemed to have the capacity to see each stage of the proceedings as they unfolded. According to my log book my largest HSP drop was a 'grader' at 35000lbs. Heavy drop trials could at times seem to need a little help from 'upstairs' as is the nature of trials they were always on the edge.!
As for the ALM going aft to cut the risers IMHO he would never have got there.
Once those main chutes deployed (10 secs I think but hopefully dragartist can help here) I think the inside of the a/c would have been opened up like a sardine can. If he had got there his survival knife would have made no impact on those very thick risers. On ULLA drops we carried a fearsome 'homemade '(in the JATE Workshop) saw of a knife to just cut through one riser.
As smudge says it is a wonder we at the coal face did not have more problems especially in light of what I have learned from dragartist, who hopefully will be along soon to explain. Ignorance for the crews was certainly bliss !