I would say that long press to stop is a feature that exclusive to Poweramp. Having the ability to stop+rewind a song is always bonus, but for v3, it is removed and moved to the ''previous song button'' which you need to enable it in the settings. For me, i would say long press is more comfortable and faster. But from a general perspective, i think there are very less people even know about this stop function existed.
Your phone device runs the analog signal through the usb-c, with that adapter, hence an effective aux slot... It works normally. My Mi Mix2S is the same case. Try using your ''dac'' on other usb-c device, you dont get sound from it.. it is not a dac anyway lol. Just an adapter.