When I did some consulting with a powder manufacture (the maker of IMR powders) the chief contact for me was the head of R&D. He was a PhD chemist. I mentioned the garden disposal idea to him. He assured me that it would take 20 years for the powders to break down. I have always assumed he was right.
As for what to do with the powder. I would start a few jugs to hold the good reclaimed powder. From the ammo, you have an idea as to what speed range the powders are. I would label the jugs accordingly and then try to fill them with reclaimed powder.
I would then take a jug of powder and calibrate it by shooting the appropriate type of load with it in various powder charges (starting low) over a chronograph and then make a note that it is 3031ish, or 4350ish. etc.
If you can get 8 to 20 pounds of reclaimed rifle powder, once it is shaken up and calibrated, it would be useful powder. I would then destroy the powder, about 45 grains at a time....