In my opinion, Mackenzie is telling the truth. In fact, Chynna Phillips half-sister of Mackenzie, back her up . Here's some interview with Chynna from LA times.
She tells Us that in 1997, 11years after MacKenzie says the affair ended, Mackenzie called Chynna and told her about the physical relationship.
"Was he actually raping her? I don't know," says Chynna. "Do I believe that they had an incestuous relationship and that it went on for 10 years? Yes."
I think that would be enough.
Know more about Michelle and Chynna Phillips.