Grant land as bribes for the influential Gaelic lords. This ought to curb any petty squabbles. Also to save face, you may need to reconsider Ewen's position as ruler within the Isles.
Leave Ewen in control of the Isles and get on with more pressing affairs in Norway. He, if anyone, will find a solution to the problems. Anyway, you need to turn your attention to affairs in Norway.
Attacking the Scots is a little hasty. It is better to wait until you're confident that your position is secure in the Western Isles.
Which card will you play?