Maybe DB didn’t parachute out but, as an aircraft engineer, knew where to access a pressurized hidy-hole in the plane itself. He hides there and then slips out dressed as an aircraft technician, FAA inspector and just walks away.
This wouldn’t explain the money that was found. Sure, it could have blown out the plane but that must be considered an unlikely scenario imo.
This new tie evidence makes more sense than any of the previous theories. Getting laid off is more than enough reason for someone to pull something like this.
Toss a bit of money out and everyone is concentrating on where he or money landed. On the ground. Not in the sky. As you said, he hides and walks out dressed as an inspector with a big bag of money and no one’s the wiser.