I think the difficulty with having Keith make that decision in Ewen's favor is that Keith and Ewen are both so invested in the ethos of "I could not love thee, dear, so well, loved I not honor more." If Keith chose Ewen over honor/duty, Ewen would likely think less of him: duty and honor are supposed to come before family, love, romance, everything. And Keith too I think would have a hard time justifying such an action to himself.