I thought that for awhile but more recent events have almost killed that theory for me.
First of all, no forced entry does not mean someone did not enter. There is no proof all doors were locked; Ron claiming he locked the doors doesn't mean much considering someone else could have unclocked them. If that's the case Haleigh may or may not have gotten out, but certainly someone else could have gotten in...someone who knew one or both kids were sometimes left unsupervised (as I suspect they were).
I believe one or more of the players would have come clean by now if this is what happened. Anyone who lies outright or lies by omission has some kind of motive to do so and the longer they keep that up the stronger their motive appears. To me, the longer they don't tell, the guiltier they look.
JMO