"Indeed you don't believe that yourself" is a perfectly acceptable translation. One notes that the grader omits a translation of "ja" -- a word that by its placement emphasizes the implicit anomaly of the speaker's disbelieving what he expects or wishes the hearer to believe.
Another word for "ja" in this case would be "doch". Es handelt sich also um ein die Aussage verstärkendes Wort.
No. "Even you do not believe that," would be used when the person accused is trying to convince someone of something they obviously aren't convinced of themselves.
"You surely don't believe that," should firstly be "Surely you don't believe that." It is used when someone is outraged or astonished with the fact that someone believes something is true when it seems unlikely.