As far as comeback player award goes....Peterson's career was not in jeopardy. Nobody expected 2,100 yards rushing, but you knew he would come back from it. Peyton on the other hand...his advanced age and 4 surgeries had people debating whether or not he'd ever play again.....
As for MVP...again, Manning changed the entire culture in Denver, given he took over a new team, with new offense (he did add some of his plays from Indy), and elevating his team to be the favorite to play in SB.
Just look at the pre-season odds of Vikings and Broncos to represent their respective conferences in SB.....I'm willing to bet that knowing Peterson coming back didn't affect the Vikings odds as much as the Broncos odds were affected after signing Manning.