There are snags, though. For a start, Cersei realises she did not consider the reaction of the smallfolk, who love Margeary, which is a fairly amateur miscalculation. She's a bit surprised when Margaery herself figures the whole thing out and calles her a vile, scheming, evil bitch, an epitaph worthy of an Eastenders villainess. Neither does she anticipate the fervour of the Septon. Instead of just believing Kettleblack and letting him go (so Cersei can send him to the wall to kill Jon before giving him a Lordship), he whips him until he blabs about not only this whole scheme to set up Margaery, but also the one where Cersei had him assassinate the previous High Septon (for knowing about her and Jaime). And now Cersei is locked up in the Sept just like Margaery. It's hard not to laugh. She's facing trial now, too. And since she has established that only a Kingsguard champion can defend a queen, she can't even deploy whatever Frankenstein's monster Qyburn has been cooking up for her (with the over-sized armour; is it a resurrected GREGOR?)
Now she's relying on Jaime. Maybe he can fight better with his left hand by now, or maybe he can recruit someone quickly into the Kingsguard (after all, there is a vacancy since one of them died in Dorne and I don't think Cersei knows that yet).
But there are already other socio-political consequences of Cersei's arrest:
- The other Kettleblack no longer commands the city watch, reducing Cersei's influence there.
- Qyburn is dismissed from the council, reducing Cersei's influence there.
- Ser Harys Swyft and Grand Maester Pycelle are in charge of which documents Tommen attaches his royal seal to.
- They have asked Kevan Lannister to come and rule.
- Mace Tyrell has stopped besieging Storm's End (good news for Stannis).
- Lord Waters, in charge of the navy against Jaime's advice, has taken all the ships, probably to join Stannis (good news for Stannis).
- Taena Merryweather (whom I'm pretty sure Cersei should not have been trusting so much) has disappeared back home with her husband.