I've had a breakthrough!
I too have been plagued by rear brake dragging for a while, but just this very morning I had an epiphany. My left rear caliper has given me problems ever since I bought this car brand new ('11 6MT JSW). It's currently on its second replacement caliper in ~126k miles. A seized parking brake cable took out the first one along with the bearing. I replaced it again very recently due to carrier corrosion (pads were stuck very badly) and was frustrated yet again to find it dragging (very hot after commute to/from work, 28 miles.)
I first verified it's not a hydraulic issue by cracking the bleeder and still having problems turning the hub/rotor. I found relief for a few days after cleaning the brake grease (applied by me) and sand that had built up around/beneath the abutment clips and pad ears (I don't put grease on the pad ears, no need to attract dirt there,) making sure there wasn't an excessive amount of grease at the end of the slide pins (hot grease hydraulic action is supposedly a thing); I even went as far as completely cleaning the end of the pin and inside of the hole to make sure there wasn't more grease than needed. The re-manufacturer had gobbed in a lot of blue high-temp grease. I had also completely wound back the piston which seems to have made the biggest difference and is the whole point of this post! I had done the parking brake pull/release several times afterward to get it back into adjustment, by the way.
By taking the parking brake cable off of the caliper and manually actuating the parking brake lever while rotating the hub/rotor by hand, I found that when the lever is completely extended/released, the parking brake is actually making the caliper squeeze the pads (slightly)! By engaging/pulling the lever just a touch (a millimeter at most, I suspect,) the parking brake lets go and the hub/rotor spin more freely. I was under the blind assumption that when the lever is completely released, the parking brake would not be squeezing the pads at all.
Looks like it's cable adjustment time...I hope that both calipers release at the same amount of cable tension (which will undoubtedly change as the cable stretches over time.) The right caliper does not seem to have this issue, at parking brake released, it spins quite freely. Maybe this is why the newer models have electronically actuated parking brakes.