This may or may not fix it, but the hook on the caliper is suppose to hang low, which makes the cable pull in a more straight line and increase the range of travel. This means the caliper pistons are on the wrong sides. No you can't just rotate the pistons to change the hook positions. It's easy to mis-assemble the rear brakes when taken apart.
And yes, you have a ton of adjustment there, pull the handbrake up 5 clicks, loosen both jam nuts create some space between them and the caliper frame run the outside nut down firm, use 17 mm wrench and continue to tighten until you can't turn the wheel by hand, now you're making progress. Both sides, test for complete release with the hand brake lowered. Trial and error, tighten the cable as much as you can without inhibiting the ability to full release.
