It is not quite as simple as that.
1st, there IS an API to control Spotify from Unreal:
2nd, the permission stuff is far more complex than you suggest. That is why Spotify is so appealing.
Anyway I am trying to get clarification (from Spotify) right now if controlling spotify app from within the game is allowed or not.
Their wording in the documentation is very unclear to be honest. Their talk of personal use refers to not end user listening in personal use (vs a Karaoke bar, broadcast or what not). Commercial apps seem to be ok as long as the commercial part is not tied around Spotify and the Spotify related stuff is not charged for.
I will update here I f I get some kind of response from Spotify.