To be fair, she wasn't dead when she made these tapes. Although, if you know you're going to die, why not say the truth? Then again, some people do believe their versions of the actual truth even unto death.
Why she didn't mention her relationship with Zack certainly is a mystery.
It was not only Hannah in that relationship, Zack was in it too. She did not want him to suffer or carve him out from his friend circle just because he dated a girl who had too many rumours about her. The reason why they broke up was bcoz Zack did not want their relation to be public. Actually he didn't want her to be anywhere around his friends to keep he safe from the fate that happened in that room. She never understood that and thought that Zack was afraid of telling his friends that he is dating her, and all the rumours about her were no less. At the end she didn't wanna screw the life of the guy she really loved. Clay was her ultimate "soul mate type" but Zack was a good boy friend. She was IMO trying to protect him.
Hannah wasn't as sweet and innocent as you want to believe. She made horrid choices time and again to fit in. She never learned her value or accepted it. She wanted it from others. And yes the truth is out. Go back to tape one where she fully admits she is doing it so they feel guilty. Suicide was another rotten messed thing she did, so she took people with her on the ride to not have to face it was her choice and hers alone.