@Lightbrand if it were ONLY a fight between Almus and Dibel, you'd be right. BUT Almus is now ultimately aiming to be the next king, since he's marrying the princess, which means he can't just beat the cruel and cowardly Dibel into his place as doing so might drag other nobles into the mess and create a full blown civil war, or at the very least the other nobles he'll have to rule over will be afraid he might do the same to them without reason one day. Almus NEEDS to have justification for conflicts with Dibel, which is why he couldn't just march forth and invade Dibel's land first even if he'd be doing it to protect the fleeing citizens of Dibel's territory. that said, it may have been by accident, but now Dibel's men have attacked Almus's troops, so now he can "justify" it by pointing out he only brought his men in response to Dibel's army marching on his border, and only attacked after Dibel's men did first. It might sound stupid if you only look at it as OP MC holding back, but it's warring era politics, so it's necessary in the long run.
as for your idea of Almus using the current king's authority to help curb stomp Dibel, remember that while the king might not have liked it, Dibel was a sufficiently major noble that the old plan was to have HIM marry the princess. also the overall power of other nobles vastly outweighs that which the king holds, remember historically kings have a higher degree of *authority* compared to other nobles, but they rarely had that much more land or wealth than any other particular noble, so, if other nobles group up against them, the king's authority means a lot less or potentially even nothing at all...and some of those other nobles are even on good terms with Dibel in spite of his nature, so they'd be faster to speak up on his behalf over the "citizens of Dibel's land" being "stolen" by Almus (again, it's kinda warring era rules, if you are a peasant you can't just up and move wherever you like without permission from your lord, no matter how much of a sack of crap your lord may be...or rather all the more so if your lord IS a sack of crap you want to flee from)