Combining theories with a bit of my own thrown in...
Zack is Dana. He was kidnapped by Mary Alice & Paul and raised as their own. His mother is the dead woman in the trunk. His memories of killing Dana are a morphed creation of his imagination and or brainwashing on the part of his "parents." He does have vague memories of a traumatic incident involving blood and death, but this is actually the death of his mother. He has been led to believe that he did the killing and that it was of a sibling so that he would be less likely to come forward with the information. Why would he turn himself in right?
I'm still a little fuzzy connecting with Mike though. Perhaps Zack's mother is the woman Mike has been looking for (something I've believed forever no matter what other theories we are going with). They were young sweethearts, first loves perhaps. He left her (not sure the circumstances). Maybe, they had a pregnancy scare, he thought she was going to abort it it and couldn't deal, but then she kept it and he never knew? I don't know. Still working on that part...