First of all, never lean back.  The only time I can think of when this may apply is when riding a short board or going to slow in deep powder, and even then I like to think of it more as "weighting the tail of the board" which is much different than "leaning back".

On steep trails, you are being pulled by gravity HARD.  You are right, if you try to stop or slow down too suddenly, you will fall.  If you find yourself in a slider or tumbler wipe-out, the best thing to do is to try to get your board back under you, back up and then slow down gently or steer to safety (or if applicable just keep ROCKIN!).  You do not want to try to use your board to try to stop.  It is much less efficient than trying to slow don't when actually on the board and besides, that is what most likely got you in this situation to begin with.

1) Make sure you have enough "forward lean" set on your binding highbacks.  Try different settings until you get a balance between performance and comfort.

2) Edging on the heelside moves your butt backwards (sounds funny but it is true) which tends to shift your weight backwards as well.  To counter this, tilt your upper body forward to compensate.