And, you know, blatant treaty breaking tends to make people feel guilty and susceptible to bullying.
I don't know the full details of this treaty, and I'm going to state potentially a biased opinion, but Carelia has had plenty of time to honour their own side of the agreement made with Suville, which was in the form of land I believe? Additionally, there are not many realms or nobles on Atamara who are fond of the snakes and now that Carelia has elected one, I'm surprised there isn't more uproar from those nations who have a large religion base that view MS as evil.
From what I've experienced, Carelia's new leader seems extremely reasonable, and a nice guy actually, but this doesn't mean he isn't a snake worshiper, which is a shame as Saeculo doesn't dislike him personally. Not compared to the likes of Regulus and Sigurd.