The point with the scars on his hands is that when he changes the timeline HE is the only person to see/remember the changes/ differences.
When he goes back and changes the past to get the scars the future he arrives back at should have been altered so that he had the scars his whole life.
Therefore the guy in the cell would have already been aware of them - not "watching them appear"
They ignored that so that he would be "converted" and help out. Ignoring the films own ideas for the scake of a plot device. Or in other-words, Bullshit.
The butterfly effect was an interesting idea, but badly presented and self contradicting.
As to Kutcher, I find it easier to believe in time travel than I do Kutcher as a grade A student: the guys a buffoon.