Do you think Macduff believes that Malcolm and Donalbain were responible for the death of their father?

No, I think that Macduff is onto Macbeth quite early on. When Macduff is required to go to Scone to witness Macbeth's coronation, he instead goes back to Fife. He also questioned Macbeth on the night of the murder why he killed the guards so haphazardly.