Even by Cersei standards, this was a dumb plot

Ser Harys had been thrilled by his appointment; So long as she had Ser Harys in hand, Kevan Lannister must needs think twice about opposing her; And for her Hand, Ser Harys Swyft; His daughter was her uncle's wife; too dim to realize that he was more hostage than Hand; I'm the Lady of Casterly's Rock; I'll order my bannerman to stay at the Red Keep