Question 1:
I'm not familiar with the GZ250's brakes yet, but from my experience with other vehicles it is not uncommon for single piston caliper disk brakes to have uneven wear between the pads. However if it is as you say one completely gone and the other hardly worn at all, your caliper may have seized. This happend to me on my car. There are usually two bolts or shafts that each half of the caliper can slide on to squeeze the pads on both sides of the brake using just the one cylinder. If by corrosion or other means the caliper halves no longer can slide on these shafts only the one brake pad gets used up. Usually when this happens a mechanic will tell you that you need a new caliper because the labor to fix your existing caliper would be more expensive.
I'll be working on my front brakes sometime within the next few weeks. I'll keep my camera handy and I may be of more help then.
Question 2:
To my knowledge, there really isn't anything that pushes the pad back from the rotor. Disk brakes usually always have a small bit of drag to them. It's as simple as when you release the brake lever you release the hydraulic pressure on the pad.
There is some good information on brakes in the how to section:
http://www.gz250bike.com/viewtopic.php?t=100