Hello and thanks for using JustAnswer.com , I am Serge and I will try my best to assist you.
It seems that you have already tried using Yahoo Sign-In Helper? If not try it: https://login.yahoo.com/account/challenge/username
If it doesn't work for you, we also can use Remote Assistance, so I can do it right on your PC, please let me know.
Here is the note from Yahoo:
"When you register for an account, it's up to you to provide the most up-to-date and valid recovery info. Your best option to avoid losing access is to update your recovery
info if you get a new phone number or change your alternate email address."
"To protect the security of your account, if you weren't successful with Yahoo Sign-in Helper or with the other options above, you likely won't be able to recover your
account. If you remember your sign-in information later, you can come back and try again."
Link to above notes here: https://help.yahoo.com/kb/SLN2051.html