Mark Jones. Interesting question re practice air-starts by diving.
This was important in Tiger Moths which were not equipped with a starter motor.
There should be no requirement to carry out this exercise in Cessnas, Warriors etc because they all have starter motors.
If the starter motor is inoperative when you sign for the aeroplane you should refuse to accept the aircraft.
There is no doubt that propellers have been known to stop during stalling practice and that is exactly why you need a serviceable starter motor. I had a propeller stop in a C150 during a low level gliding turn and it was only the starter motor that was between me and a ditching.