Fascinating story and one to serve as a warning to us fliers!
Am I reading the flight correctly that you took off from Bluff Lane and then walked onto the driveway of the house seen with the red and white cars parked up, which is about five feet higher than Bluff Lane.
Definitely RTH activated by low battery @18.21.8 but I wonder why it overrode your shut-down signal from the LH stick?
Per Meta4 above it apparently takes 3 seconds to shut the motors down and RTH initiated after I held it down for less than that. You understood correctly. It was very unlucky timing but ultimately if either I made sure that the P3P never flew under any obstructions AND/OR DJI SW didn't inititiate RTH when the drone is on the ground would have prevented the crash.