Just my thoughts -
Since it seems they had absolutely no idea who had done this until they went through that video, and they couldn't find her until he led them there, it makes sense that they're now interested in a longer timeline. Now that they have his story, however dishonest, her body, the car, and items/evidence from where they found her, they have additional work to do and were essentially just beginning the day they found her. Interviewing other people who know him or may have witnessed or heard something and were afraid to talk, or unwittingly aided him, or even intentionally aided him, searching his home and the surrounding area (sounds like they've now done that and not sure why didn't happen Day 1 when he owned up), and who knows what else depending on what they found at the scene, and what else was revealed through autopsy.
I'm sure they're interested in whether or not she was transported to another location or locations before being taken to the cornfield. They know whether or not she was killed there or somewhere else. They may (hopefully) know what injuries were inflicted prior to the wounds that took her life and where (abduction site, in the vehicle, etc) and how long she'd been in the cornfield, in the event that she wasn't taken there that night.
Side note - I needed some science credits years ago for my degree program and I took an entomology class and one of the guest speakers during the semester was a forensic entomologist from the department. He's been an expert witness in a number of murder cases and what can be known by insect activity alone is astounding. (that's already been brought up I think) He worked on a case where the victim had been killed and moved across state lines and then left on a couch in a basement, staged to look like it all happened right there, and he was able to verify where the first crime likely was because insect activity on/in the body defied the staging and the species was not indigenous to this area. And that's just one area of forensics. So, whatever they found there in that cornfield and in the condition of her body, and perhaps his trunk or his home, may have given Mollie a very loud voice, even if other things were lost through the passage of time. I am interested to learn whether or not he took her to his home or that property, or spots close by that he may have used for other assaults. This was presumably a very bloody crime so any movement of her body and any blood on him would be very difficult for him to just wipe away. If she was bleeding in that trunk and he was moving her around, who knows what kind of information he stupidly and thankfully gave police.
I think this could have been a relatively quick crime where she ended up in the field soon after her abduction, or the cornfield was punctuation after other activity. If there aren't that many moving parts, maybe what they're now seeking is what was going on with him after the crime and what does that tell them. I shudder to think of what she endured, even if much of it was terror. I can't stand the sight of him and wish media would stop split screening her image with his. It's unfair to her and her family but it's always done.
