Short term I think contacting yahoo support is the only solution.
Long term I recommend spending the few dollars per year on a vanity domain for your email. My last name is Stockbridge, and all of my personal emails have been going to an @stockbridges.org email address for the last few years. My dad set it up, and it just forwards to whatever you like. I have previously used an account from my school's alumni association, but I have recently moved to an @icloud account, and no one else can tell the difference.
I also know that a lot of people love using the web based email, but I am not a fan, and one of the reasons is offline (or locked account) access to mail. If I lost access to my mail account right now then I could change the forwarding address, and be back in business in a few minutes since all of the old messages are saved by my desktop mail client. Even gmail supports this although it seems I am in a small minority that access gmail via their imap server.