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.
It was very well planned. As far as well-executed goes, that is perhaps another situation altogether.
The original plan had been in place long before Brom lost his duchy, through the manipulation of the Queen. One of the main reasons things were a bit sloppy was because plans had to be rushed due to the Queen causing problems and usurping power from Brom and Loathin and their supporters. Those supporters were needed for a successful rebellion and reformation of Pian en Luries. The plan was never to have a real "rebellion" anyway. If it wasn't for some OOC reasons Brom would have never lost the duchy he held anyway, and everything would have succeeded the first time, with or without Amaury support. Though, as far as both Loathin and Brom could tell, Amaury was so unlikely to grant his support that it was never worth offering him to join us as he would instantly tell the Queen no doubt.
At any rate, that is all in the past, I'm just trying to clear up some things because my characters was one of the oldest of PeL before being kicked out. (And he was involved in everything secretly)
