I just noticed this thread and love this show, also!
I agree there might be a connection between the girl, Dierdre, Mike is searching for and the Dana thing.
This is what I'm thinking now:
Dana: the name of the "sister" Zach thinks he killed
Diedre: the girl who Mike is searching for, hired by her father and who he used to date a long time ago I think?
Zach: thinks he killed his sister, Dana. I think he thought Dana was in the trunk buried in the back yard.
The trunk: tossed and when found it contained the human remains of an adult female.
Mary Alice: Used to be Angela in another place and looks like she may have worked in the medical field.
Mike: searching for Dierdre, recently revealed he wants kids (foreshadowing?)
All that has me wondering if somehow Dierdre might have been pregnant with Mike's kid when she left, somehow hooked up with Mary Alice when Mary Alice was Angela, had Dana who could be Zach, who might be Mike's kid, and was somehow killed and ended up in the trunk in the backyard under the pool. Lots of holes I know, but it'll be interesting to see how the pieces get filled in.