Let's be real: Maretonia was unstable, it's survival depending on a carefull balancing act from the queen between all the noble factions, and change would have required a price to be payed in blood anyway.
What makes the assassination of the queen (which was at best "a victim of the circumstances, limited by her own role", and at worst as much a supporter of the system as everyone else) that much worse than our financing and supporting a VIOLENT uprising of the abolitionists?
Yeah, we were trying to slowly change things through intrigue, but that would have certainly included plenty of nobles' deaths at the very least. There was always a decent chance of the queen not supporting the abolitionists position even once they were strenghtened, which would have led to civil war eventually anyway.
A completely peacefull and bloodless revolution was never even a possibility. Really, it rarely ever is!
Mareia was, from what we know, mostly interested on mantaining the status quo, which mean her on the throne and the nobles mostly satisfied with her rule. If possible she would have liked to centralize power, but it would have needed to be a slow thing to avoid upsetting the cart too much.
We might have been able to use her ambition to our advantage, but she has never been shown as "good". Just clever enough to understand it was in her best interest to not give us a reason to bring our army to them.
Also, IF it was Canterbury that acted than we can reasonably expect them to have had access to some prophecy of sort. Otherwise why would they have acted RIGHT NOW?
..Actually if they plan to take advantage of the kingdom's confusion, why haven't they come to talk with us and see if they can make a deal about a possible war of conquest? We'd be at least willing to listen I think.
At this point I expect we'll get a chance to discreetly ask them ourselves. We could send Gawain, and whenever he gets a chance to ask Chevaline in private he might bring up our suspicions. They don't even have that much reason to deny it, really. If it was them I'd be perfectly fine with dropping the investigation, or try to frame one of the other noble houses.
Or at least that the situation didn´t yet warrant that kind of cloak-and-dagger stuff
well, it wouldn't have warranted it...if they had known we already had plans in motions. Which they didn't.
If they thought we were leaving Maretonia to itself, limiting ourselves to preparing for a future military conflict...well, seeding chaos by killing the queen might actually have been meant to be a way to help us, by facilitating a military offensive.
They didn't know we were actually trying to AVOID the conflict by strenghtening a sympathetic internal faction