It's possible they considered taking her body to some distant location or to get a bogus DNA sample to plant in the house or on her body. That's why the RN mentions an arduous journey to deliver the money and the possibility of an earlier exchange. This give the Rs freedom to go out all night. They could say the kidnappers told them to go out to the countryside but something went wrong and they killed her. They didn't carry out this option, but they may have gone out for some other reason. If anyone finds out and asks why they did it, they can say in the early morning hours they were looking for JBR.
The timing is the hole in this theory. They would have had to discover the RN right after the intruder left it late at night. This is a problem with the entire RN. The note says they'll call "tomorrow", but they got home late, so I don't understand why the RN doesn't say "in a few hours" instead of 8am tomorrow.
Sorry, but your first theory doesn't hold water for me. "They could say the kidnappers told them" how? I realize the lost phone records are a big hole in this case, but the Rs had no way of predicting there would be no record of incoming calls that night.
The "looking for JBR" excuse has the advantage of not being subject to contradiction by electronic records, but as you yourself point out, it would have required the Rs to claim they found JBR missing in the middle of the night--which would be tantamount to admitting they were up at or around the time she disappeared. This brings them "closer" to the crime when they were trying to distance themselves. (AND they risked their daughter's decapitation, not to call 911, but to search aimlessly in the dark.)
And all of the above assumes the Rs were willing to dump their daughter in the wild. I realize she seems discarded in the WC, but she is still at "home".
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As for the "tomorrow" reference, I know people don't like to hear this, but people have a tendency to think of "today" lasting until they go to bed (regardless of the hour) and "tomorrow" beginning the following morning.
For years I taught 8 a.m. classes at a university and I was sent countless student emails between 4 and 6 a.m., signed "See you tomorrow." Even though they were going to see me in a couple of hours. They had been up all night completing their assignments, so in their minds it was still the preceding day. Then they took a quick nap and awoke "the following day or 'tomorrow'" to come to class.
Very few people, aside from crime reporters and scientists, think of a new day beginning at midnight, because that isn't how most human beings live their lives. (Notice that we still say the "day dawns", even though that isn't the way we set our clocks.)
If, as I suspect, the writer of the RN (surely PR, imo) was still up from Christmas day, she imagined 12/26 (the day the SFF was to call) as "tomorrow", even though, technically, the date had already arrived.
(OT, but I also think this is the simple reason why the Rs put 12/25 on JBR's tombstone. They probably also liked the symbolism of the child dying on Christmas Day, but I think they would have used the earlier date anyway.)