Your first paragraph is quite odd in that you claim to have no idea what the problem is, but then go on to explain the problem in the rest of your post. Hopefully you were a little less contradictory/more clear in your communication with Party.
Here's where I think they're at. If you either deposited fraudulently, or deposited legitimately and then reversed it, you'd be freerolling them. You could play some tournaments, and if you lose, you walk away. If you win and they simply deduct the fraudulent/reversed deposit, they are allowing themselves to be freerolled. When you basically replied that you couldn't prove that you didn't freeroll them, they decided to confiscate your funds.
Sounds to me like you have to get something from your bank or Instadebit showing why your deposit was returned, like they asked you to do. Depositing from another payment method doesn't help the situation - if your original intent was to freeroll them, the deposit doesn't change that.