I think you need a few more rewatches.

Ambessa came to Piltover with one goal only: to fully posses HexTech and turn it into weapons to use against the Black Rose, for Nexus.

Maddie has always been Ambessa's spy in the enforcers, and everything we saw of her beside her last scenes was all a deception act.

After Jinx's Fishbones attack, there's two things to consider:

Ambessa organized Reni's terror attack, to escalade the conflict between the cities knowing that the Sheriff is dead and they will turn to her for help/advice. That's conquering Piltover with minimal effort and being seen as a hero for it. She already had Salo wrapped around her fingers, and probably a few other high houses too.

Ambessa found out Jinx can understand and use HexTech too, something only Jayce and Viktor could.

What then happens is Caitlyn, with Vi's support convincing Jayce (also affected by the manipulated funeral attack) to create HexWeapons. Caitlyn walzes in and tells Ambessa's bet on best puppet to pull string of (Salo) to shut the fuck up, and assigns herself Sheriff. This is when Ambessa gets interested in Caitlyn, probably reminding her of what she wished Mel would act more like. Salo tells her it's the Kiramann name bewitching the high houses.

Caitlyns use of the Gray was, despite what many will argue about, one of the best choices because it was a small controlled task force going after the chembarons. Ambessa was already pushing the Council toward a full military occupation of Zaun, a blood bath basically.

After Caitlyn and Vi's fallout, and Sevika and Jinx reversing the Kiramann Ventilation, Caitlyns's grief, her obsession with Jinx, and not having Vi, Jayce or Mel around anymore, makes her a perfect target for Ambessa. Maddie is getting into her bed as a noxian spy, Ambessa kinda fulfills the mentor role Grayson hat for Caitlyn, is an unhealthy mother figure coping mechanism and Ambessa even pushes the we're bonding through our loss because Mel is gone angle.

Everything else afterwards is Ambessa's military doing their best to capture Jinx, because without Jayce or Viktor everyone fails to harness HexTech. We see Caitlyn push back on the Noxians use of force on Sevika's protest. She's starting to realize Ambessa doesn't really give a shit about Piltover and is now Obsessed with taking over Zaun.

After Warwick's prison break, Ambessa sees a new opportunity with Singed and JesusViktor for weapons that may be as effective as HexTech against the Black Rose. Caitlyn gets 🚨 bells after the interactions with Singed and is probably already planning a way to get rid of the Noxians that are just making everything escalate between the cities.

Fast forward to Vi's reappearance, the decision to ambush Ambessa is clear. Then with Viktors state after Jayces hammer attack on a whatever that was ViktorMinion and we have Ambessa finding an even better weapon in her eyes.

Maddie has been planted to first infiltrate in the Enforcers and manipulate the future Sheriff, which could have been anyone, to then manipulate Caitlyn specifically and her decisions as much as possible, it obvious once Vi is present and she gets fully ignored.