I know wife-killers have hauled their bodies for miles before, but it's really not that common from the cases I've read....most bodies are found close to home....I can't see Joshy wanting to drive a body for hundreds of miles.
What if he went somewhere close during the 5-ish til 8:30pm timeframe to prepare a place for the body - or scout it out to be sure it would be safe to take one to that place? (for some reason I believe it's related to water...IDK why.) He could have gotten the windburn/frostbite at this time doing his 'prep work'.
Then he returns to the house around 8:30, puts the kids to bed, waits til they are sleeping (and maybe even Susan is sleeping) and does the dirty deed.
He is very nervous & in his haste to lock or unlock the van, he hits the wrong button & the alarm sounds. Puts Susan in the back, goes back into the house to get the sleeping boys, one at a time, making sure he doesn't wake them.
He drives off into the nite w/ the boys still asleep, takes Susan to the site he picked out earlier, and his hands get even more damaged.....then he drives away still nervous about what he's done....he drives & drives & drives -- no place in particular....pulls over to sleep in an isolated area & when the boys wake them up he tells them they are camping.
JMOOOC as I've been thinking of scenarios that would explain him leaving the house 2x, putting the body close to him, and explain being gone for 17 hours.