I know the USB port is powered when the ignition is on or the engine is running. So, yes - your device will be powered in those scenarios even though no music is playing.
A USB hub or splitter is possible and should be able to power and/or run both devices connected. Have not tested this myself though.
Good idea on the double phone connection. Despite being able to have up to ten phones registered to the car, it will only connect one at a time ( you can connect a phone and stream music from another according to the manual). When my girlfriend and I step into the car together, it will automatically connect with the phone last connected. If we want to connect another device we use the menu to switch.
The remaining questions I'm not sure of.
A USB hub or splitter is possible and should be able to power and/or run both devices connected. Have not tested this myself though.
Good idea on the double phone connection. Despite being able to have up to ten phones registered to the car, it will only connect one at a time ( you can connect a phone and stream music from another according to the manual). When my girlfriend and I step into the car together, it will automatically connect with the phone last connected. If we want to connect another device we use the menu to switch.
The remaining questions I'm not sure of.