if you could safely containerize the apps that you guys are asking for - in a way way and in a low latency performant way, that would be ideal.
I would shy away from any of those apps (cp/aa) running *on* my own hardware (my, meaning tesla). I don't know what is involved and how much runs on your phone and how much runs on the remote peer (the car side). all along that spectrum, things can get messy and unsecure.
I'm old school and I'm happy to run a dedicated bit of hardware (FIO, to name one) where there are NO root holes, no remote execution, no latency, no crashes, no delays, instant power-on and a very stable UI (never auto-updates itself). there is a simple wired remote control format that it supports (CPIA or something like that) and I've actually used it, reverse engineered it (if you can call it that) and was able to do basic back/pause/resume/skip control on the playlist. that covers 90% of what I'd want from the *player*, other than browsing and choosing PLs. for vol control, I'd want that entirely on the car and tesla did a great job with the thumbwheel on the steering wheel. vol control should always be on the last stage of audio, anyway (long story).
we don't have line-in, on our cars. (maybe there's a hack; I have not spent any time looking, personally). we don't have remote back/pause/resume/skip control, which is the bare bare minimum to get by. and if we had just those 2 things, we could have a 100% safe interface and we can let tesla spend their time on the driving stuff and not on the audio stuff.
come on elon / tesla: give us remote control via some safe serial interface and also good quality digital (and analog) line-in. be a mensch, give us those two things and many of us would be happy to take things from there. I would expect cool 3rd party integrations (also very safe) if they exposed that kind of public 'api', as it were.