Mail Restore request form does not work.
With no live customer service available or even an e-mail address to reach out to, I erroneously tried one of the fake phone numbers listed online to try and solve the problem. :( I am trying to fill out a Mail Restore Request form and the YID doesn't match what I have signed in with... it says. My husband accidentally deleted messages from his phone (and also the server due to his phone settings).

Here’s an article regarding finding missing emails: https://help.yahoo.com/kb/mail-for-desktop/sln4538.html
You can request a mail restore here: https://help.yahoo.com/kb/SLN2552.html Once the restore is complete you would need to check your trash folder for the recovered messages.