First off, your saddle can be at an ideal height for power and comfort regardless of the type of pedal you're running, so let's take clipless pedals off the table for a moment because, quite frankly, your post raises much more fundamental concerns in my mind. Stopping with your feet Fred Flintstone style suggests to me that you're riding terrain that's over your head from a skill standpoint and/or your bike is not in good working order. Both of those problems absolutely need to be addressed long before you ever get to clipless pedals.
My first suggestion would be to get yourself to that not-so-local LBS and have them give your bike a good tune up. If you need better brakes, get them. While there, try on some helmets and get one that fits. I'd also ask them to move your saddle up to a more ideal height. It's hard to properly control a bike if the fit and set up is way off.
Once you've done that, you might do some YouTube searches for "mountain bike" and "emergency braking." There are actually some techniques you can learn that help you brake safely, but again, given your back issues and the general quality of that bike, it gives me pause that you're doing any off-road riding without some fundamental skill development first. It sounds as if one fall could really do your back in. Is there any reason you can't stick to roads while you get more experience riding your bike?
Sorry if I sound alarmist, but I think you need to go back a few steps and correct some underlying issues.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher