I don’t see the problem with Autobiography not being authentic. The writing style, for exemple, is 100% Morrissey’s: witt, twisted, original, hyperboles. Lots of sentences that were unmistakenly Morrissey. Also, his account of how he felt as part of the Smiths and his struggle with Rough Trade (which was repeated with every other record company), his version of the coût case, strike me as rather honest reflections of his feelings and his perceptions of the surrounding world. And they are telling a lot about his own personality.
I tend to agree that some things were missing. Personality, I would have liked more thoughts on his own writing process, the lyics which holding a spécial place in his heart, his various song writing partners (I almost got the impression that it always was a smooth process and that is a bit harder to believe).