So putting together the APC, the babysitter's statement, and the account of the lady on the airplane, we get (with some speculation):
Thursday, September 19, morning: Chad flies to Mesa. (Speculation: to pick up Tylee's Jeep. They are emboldened by the apparent lack of outside interest/awareness in Tylee's disappearance)
Thursday, September 19, around noon: Babysitter arrives and is introduced to Alex. Lori and Alex go to airport to pick up MG.
Thursday, September 19, afternoon: JJ acts out while being watched by babysitter.
Thursday, September 19, late afternoon: Chad sets out to Rexburg in the Jeep. (Speculation)
Thursday, September 19, early evening: Lori, Alex, and MG arrive at apartment and MG is introduced to babysitter. Babysitter notices MG has no luggage. Lori acts extremely loving toward JJ. DW is not mentioned in babysitter account.
Friday, September 20, early morning: Chad arrives in Rexburg with Jeep (Speculation)
Sometime Friday or Saturday: MG witnesses PDA by Lori and Chad at BYU-I track. DW arrives in Rexburg by car from Utah.
Saturday, September 21, evening: MG and DW tape podcast. DW witnesses Alex take JJ to his apartment. When he returns carrying JJ the child appears to be asleep.
Sunday, September 22: DW asks Lori about JJ, she says Alex has taken him to his apartment.
Monday, September 23: MG and DW leave, either for Arizona or Utah. Alex spends 17 minutes in Chad's backyard starting at 1005.
The obvious inference is that JJ died Saturday night or Sunday, Chad dug the grave by the pond Sunday or early Monday morning, and Alex went by with the body and placed it in the grave.
Vicious amorality and and cold, inhumane manipulation of others. What monsters.