If you do not have access of wallet A, then you can't prove that wallet was belongs to you. Whatever other evidence you will provide that's all are available on blockchain. So anyone could provide that to court. Only way to prove ownership without any risk is sign a message from the address. I don't know if OP mixing up wallet with address. But if you made transaction from wallet A then change address should belongs to wallet A, it will not go automatically on wallet B if you don't put address during transaction.

Can you describe which clients you had used ? And how you had lost access of your wallet A? The only perfect way to prove yourself is recover your wallet or private keys. I don't believe some other specialist could help regarding that issue without recover private keys or wallet. Otherwise everyone will call themselves satoshi nakamoto by provide evidence of related stuff without sign message. Because all transaction history stored on the blockchain. It's not enough to prove ownership.

So actually you need prove ownership of change address? If so, then you must have access your wallet or private key of that address. I think there is not such as any mark for change address on the blockchain.

So try to recover your wallet file or something related stuff that could help recover your wallet.

On the other hand you told you sent remain fund to wallet B from change address. Then why you need prove ownership of that address, as @AdolfinWolf said at end you may sign message from wallet B from where you sent fund to second exchange. That should be right way.