There seems to be an absence of churches and religion in this story, so presumably marriages are approved and listed by the governing authority of the village, town, city, or governor of a conflict region (aka margrave). Since Lysia's father and Ars are ruling nobles (with portfolio), I guess that means they authorize marriages including their own. Since the power of the noble overrides commoners, that means the other purpose** of a wedding ceremony is invalid. In that case, a formal wedding is not the actual date of marriage but is merely be the public announcement.
**...the other purpose being a place for relatives and other parties to object and provide a reason against the marriage occurring.
- = - = - = -
As for the news that eastern 3/5 of the principality under Basamak is allying with the external enemy principality Saitz should be reported back to Ars' town ONSCREEN instead of being implied offscreen like so many other things. Without Ars showing concern that his township is notified, it makes Ars look too uncaring. Another concern is that having Lord Lemaire and Colette with him means that this trip must be as secret as possible, yet a huge number of people know of the trip. The best time to attack is when Lemaire and Ars and Colette are too far away to help with battles or leadership of troops. For a story with assassins in it, there is certainly a lack of assassination attempts on vital principals. All these offscreen things make it seem that Ars is just cruising thru rulership on autopilot. I'd like a few more things appear in the story just to ground the plot and reduce the appearance that Ars simply cruises thru everything by constant random luck of having a solution suddenly appear for every problem.