I think you could easily do it during the day. Possible scenario:
Kids leave for school around 8. He drugs her coffee/tea, then when she falls asleep, puts a plastic bag over her head. She could have been dead within 15 minutes after the kids were out the door.
Then, back the van up to the house (did they have a garage?) - remove her clothing (to make it appear her murdurer was a sex offender if her body was ever found), wrap her up in something. Pick her up and put her in the back of the van (having seen numerous shots of his biceps, no problem picking her up even if she were around 190. He's an athlete who works out all the time.
Then, by this time it's around 8:30, drive the body out to Center Road -- that would take about 30 minutes from Sheff Road (this is when rigor mortis would have set in). Center Road would not have a lot of traffic on it around 9 AM (kids have already left for school, and people left for work -- although most people out there have farms). It would take less than 3 minutes for him to pull the van over to the side of the road (nothing out in that area except trees and some seasonal cabins), run to the back of the van, unwrap her, and toss her over the slope.
Then drive the van to Plankview Manor (discarding of whatever was wrapped around her body quickly along the way, and possibly tossing her phone out the window (although that could have been done the next day). It would take 20 minutes to get to Plankview manor - -he could be there around 9:30. Pull the van in (not many people around mid-morning -- people would have left for work -- plus no one would be paying attention to a van pulling in to a parking lot). Hop out and jog to the YMCA.
It's only about 1/4 mile from Plankview Manor to the YMCA in Norwich. So, he could have been at the YMCA as early as 9:30, having killed Jennifer, and only needing 50 to 60 minutes of un-accounted for time to disposed of her body and van, and arrive at the Y where witnesses could observe his presence.
