Throwaway feasibility test. It never changes your existing playlists. The only playlist it writes to is the scratch playlist you pick below (default: Remixer), and it replaces everything in it.
Results
The two manual checks: start playback, lock your phone for 2+ minutes, then tap one of the lock buttons. Do the whole thing from the Home Screen icon, then tap one of the Home Screen buttons.
1. Sign in
Redirect URI this page sends:
2. Pick playlists
3 & 4. Read, shuffle, write
5 & 6. Play on iPhone
Open the Spotify app on your phone and play any song for a second first, so it shows up as a device.
Full cycle
Read (if needed) + shuffle + write + play, timed from tap to music.