[bigendbob]

Its interesting to see that pic from the service manual. Looks like those cutouts are intended to allow the piston to be screwed back in.
I think the initial problem is backing off the pads to clear a worn disc, so i'm tempted to go the route of removing the motor gear assembly, then gently turn the shaft sticking out of caliper to retract the piston. Might have to bleed off some brake fluid via bleed nipple. The differences document seems to suggest that the ebrake will reset itself on new pads.
The only way i can think this works is that the electrics sees a current change to motor when it stalls due to brake application, so telling computer the brake is applied. The document also suggests a retension when brakes cool, so again motor runs until it hits a pre determined stall current then cuts out.
Will find out soon as one pad as 2mm left. According the the service that supposed to be good for another 14000 miles!

Well just looked at the new brake pads i have. The piston cutouts could be to engage into the pads that have small pins sticking out, perhaps to stop rotation of piston when the ebrake operates.