To be clear, using a personal hotspot with the car has no bearing on using the cradle or the USB connection. The connection is established through bluetooth. Maybe I'm not understanding what you're saying.
One thing I've noticed with the iPhone internet tethering with the BMW is .... for whatever reason, you need to be on the Personal Hotspot screen on the iPhone for the car to open up a connection with the phone. Otherwise it won't work.
On my Mac, it doesn't matter ... but with the BMW, the iPhone has to be on the hotspot screen. Once the connection is established, you can do whatever you want on the phone then ... start BMW connected app, pandora, etc.
Not sure if this is a bug in the iPhone software, or BMW bug. I did log the bug with Apple ... so maybe it gets addressed in the future.