It is possible that your account has been compromised, most likely due to a low complexity password.
I understand the steps you have taken, but I want to make sure that all avenues are covered. If you have exhausted these options, it is highly unlikely you will ever recover your account.
You should attempt to change/recover your password XXXXX Please go to the forgot my password page (https://edit.yahoo.com/forgotroot).I would suggest using a password of at least 10-20 characters, with a mixture of UPPERCASE, lowercase, and numbers (1234), preferably alternating (i.e. M1y2p3A4s5S6w7O8r9D). Also make your security questions very difficult to answer. Don't use something as common as mother's maiden name or first child's name. Make the answer very difficult, and I would even suggest throwing in a sequence of numbers at the end of the answer to make it harder for a hacker.
If you still cannot get through or a hacker has modified your security questions/answers, and you have no credit card information or billing information on record, you have no way to verify your account, thus this is nothing you can do.If you have a paid Yahoo! Plus account, you can contact their support to reset your password XXXXX (US) 1-866-562-7219 and simply verify your billing information and you should be able to reset your password that way.The problem with free e-mail services is that they rely on security question/answer combinations for verification. If you forget these and/or your account is hacked and a hacker changes the info, you have no way to verify your account. With paid e-mail services, you can easily verify your identity by verifying your billing information.
In the future, before you log in, look at the address bar of your browser to make sure you are on the right site to log in. (i.e. make sure your address bar says https://www.yahoo.com and not something like http:// 123abc. randomurl.com/ www.yahoo.com/). A lot of phishers/hackers have been using the tactic of making a page look exactly like your log on page to trick you into typing your password.If in doubt, type in the base URL yourself (i.e. http://www.yahoo.com) in the browser.You must remember that no email service provider will ever ask you for username/password XXXXX any personal information (address, birthdate, etc.).
In any case, I would suggest to take the security measures suggested in the future to prevent a hacker from getting into your account.
If you cannot recover your account, the only thing you can do is email firstname.lastname@example.org detailing what has happened and notifying them that you account has been hacked and you would like it to be closed. If any of your friends receive those "send me money I'm in another country" emails, have them forward those messages to email@example.com. Yahoo! will likely close the account down from there.
If you have tried all the methods above, there is no way to recover your account.
Let me know if you need further assistance.