Wow - I just watched a MythBusters episode that tested this and if putting a car in reverse would stop it. Admittedly they tried it at 35 and 50 mph and each time - total failure of the hand/parking/emergency brake to stop the vehicle. Also - newer cars have a fail safe system that either won’t let you put the car in reverse or totally disengages the tranny - and they tried with automatics and manuals.
A few days ago I was driving a rental car down a very steep, icy road on Portland OR’s hospital hill. I used the parking brake to control my downhill speed and to avoid locking the front wheels. It didn’t seem like it had enough power to stop the car in any reasonable distance, but it did what I needed it to do!