This is a very interesting system. Thanks for all your detailed information and for sharing the files.
The embedded cpu board I was using won't suspend or hibernate, so I am now going to use an old Dell laptop w/ docking station that became available. My plan was to use Windows to Suspend after a few minutes when it switched to battery power, and to Hibernate a few minutes after that. Then use the ignition power line to control a relay that would toggle the power button on the dock to turn it back on.
I was wondering why you weren't using Suspend any during your process, was it just to keep things simple and consistent or did you try using Suspend also and it didn't work properly? I haven't tested the Suspend->Hibernation cycle yet with RR running, so maybe that isn't going to work if music is playing.