
Regarding the use of the slide stop, consider this... The NRA course materials are designed to work for as many attendees as possible, with as many different types of equipment as possible. Students run the gamut from burly former/current military men to "frail" old men and women with arthritis and little hand strength. Equipment ranges from well-worn, precisely machined, high-end pistols to brand-new, inexpensively made, bargain basement guns. Couple someone with limited hand strength and arthritis with an inexpensive brand-new pistol, and using the slide stop as a slide release is a non-starter - they simply don't have the thumb strength to overcome the tight springs and gritty surfaces. This is why NRA promotes and teaches the sling-shot method of returning the pistol to battery. It is likely to be more often be effective for nearly anyone.
I agree with the OP that someone taking the instructor course should have sufficient ability with their own hardware to successfully complete the live-fire portion of the class. Having a stand-by gun on hand, either their own or someone else's, to use in the event of a breakage is also a good idea.
It is indeed a good idea for instructors to work with different actions and feature sets to be able to assist students as needed.