The guy who stole the hashes is not the sites dev. He doesn't work for the sites. He simply was put on teamviewer to fix some things with the server because he knows the game code very well and he screenshotted the master seed. They are simply attempting to strong arm a payment out if us. If it was a legit win, they would have been paid. It's fraud. Nothing more.
This seems like a serious flaw in your security practices. Hopefully, in your future endeavors you will apply the lesson learned here. Or is there no way to allow someone to work on the server without having access to the master seed? If that is the case, I will never use a site that uses "provably fair" again.