I agree, I think that's a huge stretch. I just don't see these parents as savvy enough to pull that off for over two weeks.
A little side note-- last night's SVU was about a couple who said their baby had been abducted. It turned out that the couple had gotten drunk the night before and put the mother had put the baby in bed with them. When she woke up, the baby was dead. She thought she had rolled over on the baby and killed it, so they buried the baby's body. Long story short, it turned out the baby died of SIDS. I have seen some people here speculate that DB put Lisa in bed with her and accidentally suffocated her. It was an interesting storyline, especially with this case going on.