In your prior setup, were you plugged into the Lightning port and charging from a USB port built into the car? If so, you should be able to do the same with your 7. The music likely is sent over the USB cable so you may not need the adapter cable. Were you connecting both the charging cable and the aux cable on your old iPhone? You may not have need the aux cable for your car. It seems unlikely that they would provide controls over USB/lightning, but require the Aux connection for the actual music.
No, I have an older car. The stereo has an aux port, so I installed a cable and have been using that. The stereo has no USB port. My 6s would detect when I plugged in the jack and it would show the music controls. With the lightning dongle on the 7 it didn't do anything when I plugged in. I assume the same would also be true for headphones I plug in using the dongle.
Are trusted devices the ONLY devices that it will send a code to, even if you have it set up to send to phone number? If so that doesn't make any sense. It should always send a code to whatever phone is linked to my number. I understand the need for a recovery code if you lose access to the device that's linked to your phone number, but it shouldn't be limited to trusted devices only if you choose the phone number method.
And I'll always be bitter about losing my music. Why would Apple ignore music? If people want to "waste" their iCloud storage backing up music, that benefits Apple as they most likely have to spring for a more expensive storage option, making them more money. I'm extremely tech savvy so it didn't even cross my mind that the music wouldn't be backed up. SO ANNOYED.
re: 2FA, no when it asks for a 2FA code, it gives you a list of choices. It will send the code to whatever you choose, be that a specific device or a specific phone number. What I meant before was, if you lose your phone (or gets stolen) and you didn't trust any devices, then you have no other way to send the 2FA code. So for me, trusting specific, multiple devices is a safer way than sending to a phone number. I usually send the 2FA to my phone anyway, but I target that specific device in the list instead of the number. It shows up as a special notification on the screen instead of a text message.
Well the music thing was happening before iCloud, when we just backed up iPhones via iTunes to the hard drive. The problem is if you had 20gb of music on your phone, it'd have to copy all that music to your hard drive... basically duplicate data that was already on the Mac taking up gigs of space. In the iCloud era it makes even less sense because it could take hours or days to finish restoring for most people.
My general philosophy on my data is, trust no one. Use iCloud, fine, but always make a local backup of your important stuff. My music is important to me also, and I have it backed up on my Mac not only with Time Machine but also with a separate NAS. I'd get cloud backup going too, because a fire would take out both my backups, but my connection is just too slow for that.