Sorry about the 4 week delay, but just to confirm: the "invalid mount point" issue was definitely SuperSU being stuck in a state of needing a binary update (but never actually popping up and saying it needed an update until I launched it directly).
Fixed SuperSU (downloaded it to the SD card and applied the zip in TWRP recovery) and then back into CM12.1, applied the RSAP Installer and since then, every time it connects to the car perfectly (in "Car Phone" mode, i.e. RSAP active with the car fully online).
Car: Audi A6 TFSI @ 375bhp, MY12, MMI touch, BT High.
Primary: LG G5 H850 stock MM6.0.1.
Projects: SGS2(GT-i9100 on CM13) + lots of Nexus7's with various builds.
eBay soon: SGS5 Prime (SM-906K), SGS4 (GTi9505), SGS4 Active (GT-i9525).