We try to prove custody of an account through email since this is our second factor authentication. I think in this case, if you answer some questions about your account, that would suffice to show ownership.
SEND A REPLY USING THE PRIVATE MESSAGE FUNCTION TO AVOID EXPOSING DETAILS ABOUT YOUR ACCOUNT
1. What is your old email address associated with this account?
2. What is the domain associated with this account?
3. What is the user name associated with this account?