I also still think it's as easy as not wanting to just kill Dany, she needs to wipe out her army. I've posted this a few times already, but just once more to see what folks think...
Cersei knows that if Dany falls, Jon (who isn't there) will be in control. Jon, and by extension Sansa and the others, are much more pragmatic leaders and will likely pull the army back, regroup, call in more banners against Cersei, create a large alliance, etc. And then attack King's Landing down the road in a battle that Cersei has no chance to win.
Her best chance is if they attack, right now, so she's trying to provoke Dany into doing it. That's why she kills Missandei, she doesn't want Dany to be patient and wait for support, she wants her to attack with everything she has, right now.
So yeah, Cersei doesn't play by the rules, but she accepted this parlay and isn't going to break the rules of it because it doesn't benefit her to.