With all the smoking I've seen from all the family members involved in this case, I doubt seriously that anyone went outside for their cigarettes (although that would be an easy explanation for why the door was opened, and why Misty doesn't want to tell Dad that she inadvertently may have contributed to Haleigh's disappearance......).
I think that Misty was not home at the time of the abduction. And I do think it was an abduction. It is very plausible that the abductor may know the family in some way, and be very familiar with their movements/routines. If Misty left the house on a regular basis to party or visit someone in the evenings after putting the kids to bed, and the abductor/SO was keeping an eye on the place, he would have had plenty of time to go in the back door (that Misty had unlocked before leaving, to give her easy, unnoticed access back into the house, so as to not arouse suspicion with the neighbors), grab Haleigh, and take off (possibly by boat).
I think Misty is lying, but I don't think she was directly involved in Haleigh's disappearance. I pray I am right, because honestly, I have become so jaded by the Susan Smiths and Casey Anthonys of the world...my first thought when I hear a child is missing, is "the mom/dad did it". Sad world.