Okay, so I believe Brian came home with the van because he knew he had time on his side. No one would know Gabby was missing for at least a week, and it bought him some time. And if he ditched the van, he'd be leaving a trail that led back to where Gabby was. The longer he could keep LE from finding Gabby, the longer he'd have to plan.
So he drove the van home, the family met at Fort De Soto and that's where it makes the most sense that they finalized any plans and said their goodbyes.
Then Mom took Brian to ATT and got him a phone, that's a fact that we know. But why did he leave the phone behind? I can only think the phone was bought as part of the plan. So did finding Gabby so quickly make the family have to do things in a different way?
I don't think Brian drove the car to the reserve. I think his parents drove him somewhere and then parked the car at the reserve as a red herring. It was meant to keep LE busy at the reserve.
So where would they have taken Brian? Bus stop? Airport? Did they get him an uber from downtown? Meet up with a friend who took him on the next leg of the trip?
I keep wondering if he's somewhere in Long Island or if he's somewhere between North Port and Orlando because that trip to visit their attorney made no sense.