Bohunr how much total time did you dry it? I don't think you can add too much water but you can add too little. If you dried it till it was hard you over dried it. I would moisten up the net batch and be very careful with the time you dry, like 2 hours, 2.5 max. It will be shrunk up when it is cooked, but still has to stay flexible or it will crumble as you say.
I make my own recipe ground jerky and use no cure in mine. I always double grind the meat, it will flow better thru the gun and double ground makes a better finished product in my opinion. I dry mine at 155F which is the max setting in my dehydrator and it takes about 3 hours to dry 5-6 lbs.
You surely could make burgers from what is left, cured or not, if you followed the directions it won't hurt you.
Sadly most of my life has been wasted, but the rest I spent hunting, fishing, and trapping...