I was intrigued by your comments that you couldn't pause an audiobook playing on USB. I too would like to do this but haven't yet. I scanned the owner's manual and found a reference to pause, but it was for Bluetooth controls. The closest I found was one of two methods:
1. Switch inputs to say, CD or AUX.
2. Power off the radio.
When I tested this the track started where it left off. Granted, I didn't turn my car off so I don't know if that will change anything.
Anyway, if we stumble across anything it would be good to share. I'm sure there has to be a way...