Bluetooth audio definitely turns off when connected by a cable. It needs to be turned back on manually after that, maybe a "smart button" can be programmed to do that?
There's no need to have bluetooth audio on while it's on cable power. BTW, I get a message that the device (iPhone 5) won't charge, but it does actually charge.
I use the "Y" cable because my iPod Classic will begin playing REALLY FAST, (Alvin and the Chipmonks style) randomly when using the Apple white iPod cable.
2013 335i xDrive – 6-speed, AW, M Sport Line, Cold Weather, Dymanic Handling, Premium, Technology, 403M, Black Grills, Carbon Caps, Performance Exhaust