Do I send my bet to the Ethereum blockchain or to your wallet?
Yes, when you send a bet to the Ethereum smart contract, croupier will automatically settle, open and transparent.
But the source code for the croupier is closed source. How do we know you have not tempered it unless you verify it or show us ?
The croupier is only responsible for calling settleBet. In theory, anyone can call it, but for security reasons, a croupier has been appointed. If croupier has a selective prize, you will find it from the blockchain.