2. And just discovered it's playable as an audio stream via mpd and the hifiberry-connected hifi.
Grab the youtube url. Assuming you have youtube-dl installed, run the incantation "youtube-dl -g <URL>"
This generates a humungous .m3u link, which plays perfectly through mpd. See https://www.reddit.com/r/linux/comments/8pqgan/mpd_can_play_streams_from_youtube/