Technically, you weren't a Duke anymore by the time the rebellion went down. You had no duchy left to throw behind the rebellion, like the three Dukes in this one have. And the main reason for the failure of your rebellion lies with the rebels themselves... it wasn't a particularly well-planned or well-executed endeavor, you have to admit. Supporting known frauds like Mordaunt, incompetent leaders like Gwendolyn, terrible food management, falling for the same traps over and over during the civil war, leaving your capital open for an attack etc. all contributed to your downfall more than Amaury could. Hell, you could probably have gained his support if you'd been a little more subtle and clever about the whole thing.
