To me it boils down to something very simple. Who would Diana have gone with willingly at 6 a.m. without putting up a fight, sounding an alarm, screaming, or raising a big stink of some kind ? Bio mom, step dad, bio dad, Guerrero. Those are our four choices. Everyone has been accounted for that morning except for Guerrero.
I think he planned this out for a couple of weeks and deliberately set things up to create confusion either in the event that he got away with it, or in the event that he didn't. Since obliterating her entire family in a house fire failed to work, part 2 of the plan was enacted, which means distancing himself from everything in whatever manner necessary, including the elimination of witnesses.
All he can do at this point is sit in a jail cell and deny everything and force the cops to prove he's the guilty party. Hopefully they will be able to do that, thus their return trip to the parents house in order to try and find additional evidence that he was there that morning. I think they have the right guy in custody, but he's going to force them to prove it. A lot of the confusion surrounding his whereabouts in the last week may well have been orchestrated by Guerrero himself.