1.The Spotify Windows 8 mobile app does not have a radio feature. This is the reason I use Spotify on my computer and used it when I had an Android phone and an iPod touch! Until this feature comes back, it is worthless to me on my phone. I cannot afford Spotify Premium.
2. Any time I try to log in with my username instead of my Facebook account, I get a login failed error: "You need a premium account" (409). I had no problems logging into my old Android phone with my username, so if this is intentional, I don't understand why.
1. You are correct, mobile radio is currently only an iOS and android feature, it has never been present in the Windows Phone application. There is already an ideas topic requesting radio be added to WP8 here if you want to add your vote and voice there.
2. You do need Spotify Premium to login on Windows Phone 8 since there are no free account features on that application.