Kevin and Wendy come up with the 'Seating Order' from nowhere.
Kevin tells Wendy about Flight 180, and how everyone died in the order they should have died (though it makes no sense how he'd know that they died in order, to me) and then later, they suddenly start talking about how it was about how where they were sitting that determined what order they died in.
As much as I hate to admit it, the "Order of Seating" may be correct - Frankie falls before one of the Ashes, I think (though in the book, it's apparently based on death, not seating). Though I really don't know why the writers suddenly made it about where everone was sitting, and not when the died.
The point is, Kevin talked about the previous victims dying in order of death, not seating, so how come he and Wendy suddenly come to the conclusion everyone dies in order of seating?
My favorite Final Destination is the 4th – DEAL with it!