Can you YouTube how to cesarean deliver a baby? Without a medical background I can't imagine thinking you can cut a child out and have it survive. Even animal births have to be sensitive to not lose the mom and babiss. If it's true you have 3 min (not saying it's not) after the mom dies that doesn't leave a lot of time for a first time "let's cut this open". I would think you would need medical knowledge or practice.
20+ years ago I lived in that area and went to school (Go Bison).You would drive back and forth between state lines without even thinking about it. They are good people, everyone will search. I hope this goes federal. I've hiked many miles in the state park indicated. Granted it was along time ago but there are lots of areas to hide a body but only with premeditation.
I boarded my horse in the area. There is lots of land to plot to hide but one would need preplan and dig. Not impossible but I don't think you are going to be able to dispose of anyone in a manure pile. Most barns watch the comings and goings and someone would have said something. No one will harbor a murder IMO. Farm fields seem like a good option, something unmonitored.
No offense but they don't seem particularly bright. Having the baby in the apt after police searches to me says they are low on the bright light scale. Keep it simple, they didn't think things through, as much as they thought they did. They thought after being searched three times it was safe to bring an infant in and the neighbors would be cool with it.
The river seems most likely, in pieces IMO.