Yeah, I was going to say pretty much the same thing, except TMO was "under maintenance" and didn't let me

The phone can easily tell between line in and headphones by the impedance. I assume the same trick is used currently to tell between headphones-with-mic and TV out, but of course I don't know for sure.

Class D amp may be a PITA but class A or AB is a power guzzler and battery killer. Class A pre-amp for line out followed by class D for speakers and headphones?