Season 7 finale spoilers.
I loved the season 7 finale. But there have been some difficult-to-swallow plot points lately. This is one of them.
Cersei has relentlessly and openly exhibited her willingness to shatter any moral norm necessary to help her consolidate and preserve power. It strains credulity to believe that Jon and Daenerys could miss this, and think that a plan that relies on allying with Cersei is viable. But to believe that Tyrion could buy into it is beyond the pale.
You have to know that she will, from the get-go, be deciding to ally only for as long as it seems to be in her interest. Her words mean nothing. Honor means nothing.
It’s just not a viable plan to rely on her for a truce or an alliance. You’re better off killing her. An assassination or dragon attack plan would be better than an alliance.