How about you follow what they have suggested and provide more concrete evidence like the one made by the forum member on the link that examplens provided. If I were you, I'd also try to prove the ownership of the email address if you did put a correct email address on the account when it was created then you might have a chance to prove the ownership. Your signed message may be an evidence but it's the second wallet address and not the first which it may also served as an evidence that it may have been given to you.
If you were the original owner, you'd also have known that this is an exchange deposit address, so you won't have the private key for it.
It seems kinda fishy if op were the original owner then he would know about it but as you have exposed it is that he is looking for the private key.