My theory for Cersei's final play:
Cersei hears them out about the dead men beyond the Wall and agrees to help, purely to have an excuse to march her armies North without being met at the Neck and stymied. As soon as she's got them in position to do maximum damage, she betrays the Northmen.
Then, she marches on the Wall.
Why? Well, it's fairly simple: she knows Jon will try desperately to defend the Wall. This is to her advantage because the Wall is almost entirely indefensible from the south; there are a dozen undefended castles he would have to spread his forces between, and none of them have any fortifications against southern attack whatsoever. He would have to spread his forces thin, put their backs to a literal wall, and fight without any defenses.
What's worse, every man she kills--and every man she loses--will just be fed into the Night King's army, which is more of a threat to the Northmen and their allies (Daenerys) than to Cersei herself.
It's horrible, it's mad, it's the best way to doom all of humanity... and it's also the most cut-throat way to destroy her enemies, which is all she really cares about anymore.