I'm new to this site and want to toss my theories in to the ring
A few posters have mentioned this was a lot of work for DW to get very little. It would have been easier just to steal one or two of the cars. I don't think he intended to be part of an overnight home invasion. Maybe JW heard SS talk finances during one of many of the daily trips; SS worked in the car while JW drove. Did JW misunderstand something and thought he heard something to indicate a lot of cash would be in the house. My guess is JW was desperate for money to fund his upcoming expensive racing trips (CA in June and Italy in July) and came up with a plan with DW.
Maybe DW was just supposed to get in and get the cash and get out. It is too risky to do what he (DW) did which involved contact with too many people outside of his control. Perhaps there was a plan to take a car or two or not. If DW is an average criminal he knows he needs to get in and out fast. There is so much risk in trying to keep several adults and a 10 year old under control over night. Even ordering pizza presented an opportunity to be caught. I think from inside info he thought there was a quick payoff and instead he ended up with so little it required a change in plans.
This may be why JW lied about when SS called him regarding the pickup of the money. JW lied and said SS called him Thursday am to pick up the money. Maybe he lied because it was DW who called or texted him Wednesday night and JW isn't a criminal and isn't quick on his feet. Perhaps DW called him to let him know plans had changed and they would be scrounging for money Thursday morning. DW could have been using SS phone. JW lied about how he got the money because he wanted it to appear that he didn't have a clue that money was in the bag. The phone calls from SS to JW that JW didn't answer while he was in the Lowes or Home Depot in Chantilly could have been from DW. It could also have been from SS if JW took money from the $40k. I will be interested to hear who got the box truck and when. Was it stolen or rented and at what time? The fact that JW texted a pic of the money means he is either innocent or the most stupid criminal on the planet.
I also agree with several posters about SS trying to escape. I don't know a dad who would leave his child with DW knowing what DW had already done to PS. Most of us don't have training in the best strategies for dealing with a situation like this and knowing that DW had already hurt his child might make it impossible for SS to act.
IMO, that last sentence is key. Even if we "have a plan" or "know what we would do", without repetitive training that allows us to use those skills if/when they conflict with our instinctive responses in a traumatic situation, I'm not sure how much good "pre-planning" does. Soldiers/fire fighters/police officers train so that they can act/react consistently and predictably in spite of danger.