a) was the main killer for me - I'll look at getting back on the Spotify train just for that. Deezer is good, but it ain't Spotify. The gapless playback is something I have learned to live without, but it would be really nice. The other two features would just be a bonus.
You should also be able to log in with your facebook account, after you setup a Facebook "app password" (in your settings, security section). This generates a special password locked to a specific app like Spotify, e.g. KDFJK347A. Then, when you log into Spotify, you use the email address that you use for Facebook, and the newly created app password. Worked for me.
It does seem that gapless playback is implemented. The new Gruff Rhys album has a short intro that then leads into the second track - and it does so seamlessly. On Deezer there was a definate, jarring gap. Very pleased with this.
On my UK based Nokia running 8.1 with region set to US for Cortana I get the following message when I start Spotify, sometimes. I can't recall the exact phrase but it is something like - "the spotify app started with some unknown options this will be ignored and the app will be started normally". The app then starts, well, normally. Anybody else get this?