Wow, that sucks. i fortunately have never had a phone crack up like that.
Not that is is any consolation for you, or that it even would have helped... but I did find a cheap silicone case on Amazon for this phone. The vendor originally sent the wrong case but corrected immediately with a case that fit the Lumia 521. I also had a beef with the 521 feeling very slick in my hands. Not so sure that slick plastic back with the beveled edges was the best design..
Anyway, if you happen to get a replacement 521, I have had good luck with the feel of this case:
Amazon.com: nokia lumia 521 case
Not quite a Siedo case, but will sufice until Seido comes out with a good case for this phone. It may be a false sense of security, but I have always kept all of my phones in some sort of skin case. Have yet to have on break on me.. yet..