
I agree with you that it could very well be about fight for control or power and WE position. But I'm not sure how much say Silva and Leon etc have right now. King was named Augustus Kira clover XIII so I think Kira is the main royal family and other are branches.
Damntio certainly figured out the whole story and had assessed Asta's disposition correctly. He is killing two birds with one stone by blaming Asta. This way he gets to save vanity of elite royals who are reluctant to acknowledge that two kids from the slums along with their misfit magic squad, saved their hide. At the same time they can blame Julius for letting a devil boy become a magic knight and cause damage to the kingdom.
Silva and Leon maybe in the same boat as Julius. Julius wishes to save William and Kira still want someone on trial and put blame on to for the mess. It's not just Julius's failure but all of them collectively failed in identifying William timely to stop the devil. Then they had to rely on another devil and elves to solve the issue. It makes them appear weak in public.