It’s not a second regen brake. It’s your parking brake; I think it’s also used for holding the vehicle if your foot is not on the brake. The regen brake is just the motors working as generators. It may be that the larger motor doesn’t use the regular brakes for this.
Vehicle hold while in Drive/Reverse does not use the parking brake; the car uses the iBooster to hold the main hydraulic brake circuits engaged just like your foot was on the pedal.
Agree with the rest of your post.