I have several proofs of account ownership:
My Bitcoin withdrawal address
My registration email
The referral link I used
The approximate date of my last login
Typically, Only KYC is the only way to recover an account that lost 2FA since a hacker can get all those supporting details by compromising someone device.
But I think a sign message on the receiving address for the withdrawal can be a substantial document to claim ownership to your account.
However, Freebitco.in support ticket is very slow to respond. Your best chance to wait CSFB account to become active again here and handle your case.
This wallet address only handles transactions above 3k FUN.
I have always received from this wallet address.