I was under the impression that using Spotify's APIs in a commercial project required an agreement with them, presumably that costs money. Have you investigated that or will you have to drop the Spotify aspect if/when you start taking money?
I think they mainly want to prevent people from competing with them; I'm not sure if they care too much about charging you money for it (I guess they probably would if you were making a lot of money).
Also, thanks for your work on Mopidy!
I still subscribe to Spotify but for anything I love I buy the vinyl/flac copy and store it on my Roon server at home.