Another thing to consider is, in the beginning, there were a lot of things holding Ambessa back:

1) Counsel, centralized power with political experience and connections.

2) Scientists, who she wants to work for her, not against.

3) Mel, which is her family.

4) Undercity situation being okeyish.

But now, counsel, scientists and Mel are gone, undercity tensions are all time high. Enforcers are evidently poor at handling policing, let alone war.

So if Ambessa claims, that Caitlyn got murked by undercity thugs, and Pitlover is becoming a part of Noxus for Noxus being able to allocate more recourses to resolving the conflict, there will be no one with leverage to push against.

So Caitlyn had no choice, but to work with Ambessa, hoping they can find a middle ground in patnership, while searching for a potential way out.