Also would ideally want to control the output volume of the pc app through the phone volume buttons itself…
When streaming from the phone to the pc, there is no need to have audio coming out from the phone…
It should work like the bluetooth audio sink.
At first glance, it should be possible. The app would mute your phone and react to volume changes / key presses. (at worst, it’d simply mute the phone again if you unmute via a volume key press)
Transmit the events to Windows won’t be a problem.