Constriction isnt learned, it is preset into them, so feeding live wont 'teach' the snake to constrict, and many snakes will eat unweaned prey without killing them because there is just no need to; they wont hurt the snake and they arnt going to run away, so why waste the energy. It is the same exact reason why many snakes wont constrict thawed prey, because they know they dont need to waste their energy on it. Now on a good feeder, give it a thawed mouse and if it doesnt constrict move it a little bit as a stuggling mouse would move, the snake will grip onto it tighter and start to constrict to restrain it.
Snakes arnt stupid, if its fighting back they will constrict or let it go. The only time I have ever heard of a snake eating an older mouse alive is when the prey was first stunned, but woken up as its being swallowed.