A few more thoughts, with some admitted repetition of above:
Get mercenaried up. Denies others access to any merc groups we have hired. Increases our retinue size. Looks like Ewen too big for own britches, maybe. Given surprisingly strong retinue size, and leaning on Prehil alliance if possible, leverage for a seat at the table which is above, strictly speaking, our station.
Once at the proverbial table, use Sir Ewen's ability to influence others to shape some lousy military decisions along the line of classic blunders. Divide forces imprudently, feed units into battle piecemeal, stage glorious but suicidal cavalry charges, encourage temporizing in the midst of crisis, etc. Focus on getting as much of the flower of Kaldoric chivalry slaughtered as possible. At the same time make noises of respectful dissent when these bloopers take hold, so as to appear wise in retrospect.
Send Cekiya behind enemy lines to betray critical intel to the Vikings. Hope she doesn't get caught.
Consider how many more companies of Thardans Sir Ewen might be able to squeeze in under wraps right under the noses of the Kaldoric nobility. Use Thilisa's life savings to bankroll as needed. Use manors, or use Thilisa's cash to obtain another building or two in Tashal, to quarter them. Lord Graver could start sending them over in groups of two and three to be assembled as full companies once in the kingdom. Why wait to get more Thardan troops inside Kaldor, why not start now?
Arrange for assassination of Caldeth now, as in very soon, making Sir Ewen de facto Earl of Vemion during the crisis. Make look like accident, with national crisis lending some obscuring chaos perhaps, but at least diminishing leisure of the king to looking into a suspicious looking death. Going to have to be done eventually, let's get it over with. Send Cekiya out to whack him, hope she doesn't get caught, or pay Escallus lots and lots of Thilisa's cash to have her father murdered. Keep it in the family.