I not sure what you mean by "McGuffin".

I've read the book and have seen part of the movie. I caught just the last 45 minutes or so of the movie on TV. I should rent the movie although I doubt that it would come close to matching the book.
The book made it very clear that this was the aftermath of a nuclear war. The survivors seemed two fall into two camps. Those that would stop at nothing, including cannibalism, in order survive and those that never give up hope that humankind would survive and never give up its humanity. The part at the end after the father dies and the son is left to fend for himself but runs into a small intact family was a metaphor for rebirth. The rebirth would run through their young daughter and this newly found little boy. Adam and Eve. It's a story of hope verses despair...good verses evil. I thought the book was a small masterpiece.
I've read most of Cormac McCarthy's novels. Three have been made into movies.
All the Pretty Horses,No Country for Old Men and The RoadI'm definitely a fan.
...Roy