You are adding the user as feepayer, which means it needs to sign, but looks lielike you are not signing with it. So the signature would be empty.
So you could probably use the authority key as feepayer which should solve it. Or partially sign with the user keypair as well.
For the second signature: Can you add the code from the createMintCNFTInstruction its probably adding another signer there or its the signature of the authority.