Interesting thread. I've always wondered whether someone could be successful installing a parallel audio system, or would even try.
You clearly say that you haven't lost the parking sensor tones, but I can't tell how you avoided losing them. If you are essentially hijacking the speaker cables for use with your parallel system, how are you channeling the car's operational audio (tones unrelated to entertainment) into your system? Or are you somehow connecting both the OEM and parallel systems to the same speakers?
2013 535i xDrive M-Sport | 2005 545i Sport (Retired and missed) | 2001 X5 3.0 Sport (Retired)