Is this only in Demo stage? I paid for game, and after tournament it says I've reached end of demo

You don't buy the Demo...it's free (and that's why it ends when it does). But if you purchased the game, you would have gotten a Key via email. Go to "License Key Management" and use that Key to activate your license...you can then continue what you've done with the Demo.

I've input key i received, multiple times, and it still says demo finished even after key was accepted.

Suggest you submit a Support Ticket.