Did you success to create your cd changer emulator?
I have a Peugeot 406coupe MY97 with a Philips radio and cd changer. Some years ago I begin to analyse the cd changer protocol to design an emulator and replace it by MP3 player. Finally I renounced because I had not success to understood its protocol.
During my protocol analysis, I did same observations as you about voltage and timing.
To record messages, I used a RS485 device to convert low dual voltage signals to TTL, connected to a laptop by LPT port, from software side, I used a small program, like data logger, running in kernel mode under Linux to save signal state and timing transition with better accuracy possible.
Next I tried to recognise a protocol, here I failed!!!
So, I will re analyse my logs with your informations. I hope I will able to identify cmd and so on... And give a feedback...