With all due respect:
Kids can come down with illnesses at any time, doesn't have to be too much candy. What does the part I bolded have to do with anything? If a child has a fever at 2 a.m., a mother isn't going to worry about getting medicine until her normal wake up time at 6? My kids always slept when they had a fever but they still needed to be given something to bring it down. We don't know that the girl went to sleep and didn't wake up again until 6, she could have roused enough for mom to realize she was out of Tylenol and then went right back to sleep, never knew mom left to run to the store.
I think it would be wise to think outside the box and not assume too much. Sometimes the simplest explanation is the most reasonable. For whatever reason, she was on the side of that road with a flat tire, someone came along and snatched her, took her to the location she was found and murdered her. I think I remember reading early on that LE believed she was headed the other direction, and made a U-turn to park on the other side of the road pointed towards home. Which could explain why she never made it to the store or wherever she was going.
It could have been the husband... or it could have been a stranger.