Rear pad change,ebrake stays on.

[ManoNegra]

if the piston turns is going in or out
the square tool makes it an easy job, pliers will work too but a pita

could be the handle/cable needs adjusting after new pads
10mm nut under your e-brake handle boot

did you make sure one of the 90° grooves in the piston lined up with the dimple on the pad?
if not, this can lead to all kinds of fail

is the piston seal nice and flat or did it get twisted as you turned the piston?
this is the reason my passenger rear started failing recently
which led me to overhauling my brakes
if it is, clean it with brake clean and use a silicone lube spray to loosing it up
avoid WD40 as it will become gunk when it mixes with brake dust
if it's beyond saving, consider a rebuilt kit
the rear kit is ~$20 for both pistons

and finally, ditch the stupid oem sliding caliper set up
and switch to the z32 piston set up