So, it's not a scam.

You're just not completed the document they're asking, It's on their term & condition and you cannot blame on them as well. Becasue it's part of their service rules, proof-of-source fund is always tricky for people like you. Your account still temporary locked and can be opened, but with condition finish the document first. If you keep asking, and not completed the document it's just wasting their resource time. Try to submit, and contact them again not just keep spamming communication.

Still cannot find the alternative for this one, I only got alternative for proof-of-address if people are still living on their parent and all the bill under other name not us. Did you pay tax in your country, If yes then tried to submit that. You never know, it can be accepted or not.