The handbrake signal is a pull-down switch. This means that if you set the handbrake cable to ground, the radio will think that the handbrake is permanently applied. So connect it to ground somewhere, otherwise screw it to the housing, glue it, whatever.

Yes, tried with S8, S9 and S20 5G, mirroring also works while driving. I did install the handbrake wiring and it required to be set with the handbrake on when I first used it (presumably so you are not distracted for too long while driving). After that you could always adjust it while driving.
I can't answer the USB question - never tried it. Radio highly recommended!