Hobo
You said,
'After PI, there was a mechanical interlock put on the levers, so the droop lever couldn't be moved without the flap lever in the up position.'
I was on my initial conversion 'sim' phase when PI was lost, but I'm sure the flap/droop interlock was in place before that accident. There had been a suggestion that some captains were in the habit of raising the flaps as soon as they were airborne, having utilised their effect of minimising the take off run, and by their retraction improving the 2nd sector climb angle.
By doing so, they were removing the lock on the droop at only two or three hundred feet.
The DC10 had a mechanical link between the trailing/leading edge levers, so that both levers moved together, and only the last selection would retract the slats. The system was (IMO) foolproof. Conversely, the Trident arrangement was an accident waiting to happen.