I am very sorry for the delay in getting back to you...but this is a VERY tricky situation you are in....and quite Unique.
Its hard to know where to even start with this one.
So....lets start with....the yahoo password.
You say its correct when you enter it in, at the login screen....but Yahoo says "Continue reactivating your account by contacting customer care."
When you contacted Yahoo customer care, they said WHAT? I dont see where you made contact with Yahoo Customer Care.
Secondly, the ATT account, which i am assuming is your Internet account for your HOME internet service.....yes?
When you contacted ATT and REestablished your ATT account, they had infromed you that your BACKUP (stored on Yahoo) information would be available within 24 hours.....but that never happened......right?
So.....i think your next step here is to contact Yahoo Customer Support.......or you could give me your PW, and i could try to login from MY side???
I am a 100% professional.