Prolonging the time between death and loading of the latest save game
Solution 1:
Seems like you want to increase the duration of the death cam, which Death Cam Duration Options can prolong. There's even an insane thirty minute version!
Solution 2:
The easiest way is to buy a horse from some stables, and to fast travel to where you want to test from. Then ride the horse off your selected cliff.
You can occasionally survive a fall even if your horse dies, so Bethesda stopped any saves from loading during your decent and only registers survival/death upon the character being stationary.
Enjoy your horse killing and character destroying.