The last thing a meth addict is looking for is a dead baby to haul around (or a live baby). If there was indeed a man carrying Lisa around, it is a relative of Deborah or Jeremy, IMHO.
I can't really see that in this case, once again, assuming the guy is the perp. It's irrational behavior, the kind you'd expect from a hard-case drug addict, or psychotic, or both. I'm not ready to speculate on an elaborate conspiracy theory drummed up by several members of a family that has really shown (as far as I can see) no predilection for the kind of actions we're looking at here, particularly at such short notice.
The authorities right now are doing what I call "playing the paradigm." They've no rational explanation for what happened; they can't break anyone down (which I think would be easy to do if any of these family members had been involved), and since the mother was the only one at home at the time, it's got to be her by default. And it's not just the police: every damned amateur sleuth out there has got the woman tried, convicted and fried. And there's only one element that's wanting to complete the case, namely, evidence.