would Ewen be open to persuasion? Probably not from Lachlan, but if Lachlan went to, say, Lochiel...

Aaargh, that is a good idea! I suppose Lochiel wouldn't necessarily agree with Lachlan entirely (I'm sure he'd be less dramatic about it), but he might have his own views on things—and that could be a really good way to bring in the contrast between Ewen's established network of loyalties, as represented by Lochiel, and Keith as an element not fitting into that, as luzula pointed out above. Hmmm...