Consider d) phones necessarily require DACs for their speakers so they'll always be able to dump analog signals onto the usb-c pins. All that's needed is an agreement of what to expect from the connector when analogue audio is playing and not fry equipment expecting something else, and best-case scenario all those pins can now be used even for balanced 5.1 audio (there are 6 positive-negative pairs free for alternate use according to wikipedia). So the dreaded adapter, while still unappealing, can be as simple as connecting pins to the correct 3.5mm pins (altho if usb requires some acknowledgment to get to that point it would only require a small finite state machine to achieve, I couldn't say what the impact of that would be on adapter size)

e; it occurs to me i have no idea the different impedance for usb signals and line level, so this could all be hogwash