The software can actually be aware of specific equipment because the software is VIN specific. It varies depending on hardware equipped on the car and Tesla is very aware of when they made hardware changes. They don’t like to share the vin specific equipment changes but they are aware. Not saying that this is the issue and they don’t make mistakes but the software without a doubt is specific. If the different equipment is retrofitted then obviously the software can’t see actual equipment.
Parking brakes work better going forward only on rear drum brake cars because of the design of floating shoes in a drum "self applying" force as the drum rotates forward. In a rear disc application the direction of rotation has equal braking and thus makes no difference.
New Mercedes vehicles also use the same type parking brake on the rear discs as Tesla instead of an old style parking pawl in the transmission. It wouldn’t surprise me if it was sourced from the same supplier as MB uses.
Software is aware of the VIN, but then there needs to be a database of what VIN has what parts. That database needs to be updated whenever a part is swapped out. Keeping an up to date database of all parts in each car is a huge burden, and nobody does that. For parts which have communication controllers, the version of the part may be auto-discovered, however I'm not sure calipers have their own comms interface.
HOLD does not use the parking brake. You push and hold the brake pedal until the HOLD indicator cones on. HOLD is released by pushing the brake pedal again or pushing the accelerator pedal.
Older cars have separate dedicated parking brake calipers on the rears (my '14 does). At some point, they redesigned the main rear caliper to integrate the parking brake function and eliminated the dedicated parking brake caliper.
What possible reason would there be for a half-assed "partial" parking brake mode? That makes no sense.
