Most say Cersei is ruthless, cunning, but loves her children. I call BS. she only "loves" her children because she gets reflected glory. Witness the "Tommen betrayed me," line when he committed suicide.
She's not particularly cunning. Most of her schemes go badly. For example giving the high sparrow so much power. Sending Jamie to Dorne to fetch her daughter back home. Sending Bron to kill Jamie and Tyrion (wtf did she expect?)
She's not ruthless, at least not directly, and only when backed into a corner. She did it through 3rd parties.
Ned tells her he knows her children are bastards of incest, so she is forced to have Robert killed and Ned charged with treason.
She has the Sept of Baelor blown up because she was going to be imprisoned again. But she watches from a distance.
She tells Jaime not to leave her to go fight with the North, or she'll have him killed. Then has The Mountain stand down.
She wanted to have Tyrion executed for killing Joffery, but not by her hand, but after a trial and with an executioner.
So to me, there was no doubt she would stand down again and not give a public order to kill Tyrion.
Also, she's had no experience with warfare. All she has is Euron Greyjoy, who while adept at sea battles has no on land experience. She has the sell swords of the Golden Company, and I'm sure their leader is skilled, and it is possible that Jamie will return to fight for her, but...
Finally, remember the Battle of Blackwater, when it looked like Stannis couldn't be stopped? What did she do? She went and hid in the throne room and prepared to commit suicide. If the Targaryen forces look like they will overrun Kings Landing, she'll kill herself.
just my two cents.