Usual clue is a hot wheel, I’ve had a couple of seized calipers in the past, so I usually check the wheel temperatures after a long drive, but it will show up after a few miles. It sounds like you need a new caliper. On my car, the offsides went first.“Normal om most cars” (not just MX5’s)said my local MOT and service garage. Another clue - if you have wheel centers, they tend to drop off, heat causes wheel to expand, so cannot retain wheel center. Most plug in. - I’ve glued mine in now, as they are expensive to replace.
When you say ‘wind back’ - do you mean you’re ‘squeezing back’, using a “universal wind back tool” to compress the pistons back into the caliper - and you’ve bent the handle on that? That’s fine for the front brakes - but you do know you have to use an Allen key to properly wind back the pistons on the rears, don’t you? You can’t just compress them back into the calipers, like you do with the fronts.
You need to remove a blanking plug at the rear of the caliper and inside the aperture beneath it is an adjuster, keyed to a groove around the perimeter of the piston. A 4mm hexagon wrench (Allen key) needs to be inserted in the adjuster and rotated anti-clockwise (looking at the caliper from the inside of the car) to retract the piston back into the caliper body.
After fitting new pads the adjuster is screwed in clockwise to bring the pads to the disc face, then wound back very slightly until the disc is free to turn. The handbrake lever travel probably needs to be reset using the cable adjuster on the other end of the handbrake cable (driver end), it’s best to unwind the cable adjuster first before working on the caliper.
Don’t forget to refit the blanking plug to seal the adjuster aperture in the caliper, noting there is a copper sealing washer under it. The latter may remain stuck on the caliper body, but it does need to be there.
Yes you take off a 14mm cover bolt at the back of the caliper not the one that holds on the handbrake cable bracket, the one beside it and nearer the centre of the wheel. You then use a 4mm allen key to wind the piston back, replace pads. Allen key again to lock the disc and back say one third turn.
In saying that, last week I replaced a rear caliper and the little adjuster that you put the 4mm allen key ingto on the leaking caliper was seized but no a problem as it was being binned.