The experience of AA really depends on your Android phone. Your best bet is to go with a Nexus phone for the least and fast-solving compatibility issues.
My experience with AA really s**ks. I am using LG G4, which has problems working with AA, confirmed by Google...
You may import mp3 files to the flash drive using Windows Media Player. The settings in WMP will allow you to create folder structures by albums/artists. And yes, you can have album art.
Just got mine working with your solution. The system was not recognizing my USB before I went into 'Settings'. As soon as I hit 'system information', it started to play the music file in my flash drive. And no need to do this again when you start your car in the future.