No, she slightly manipulated his downfall and even then it was almost entirely down to what Baelish wanted to do. Not exactly a safe bet.
Ned wouldn't have been executed if Joffrey wasn't a fuckhead.
I can understand the one above, but how do you reach this conclusion? That was all Tywin.
what
If you actually objectively look at the fact she has no subtlety and can only kill people pretty much directly (see Jon Arryn, Robert Baratheon), it's really not hard to see how she thought she was as clever as Tywin and thought she was a genius manipulating the High Sparrow.
Tyrion took away all of her power in as much time as the Sparrow and was only undermined by Tywin. The show's made a few illogical moves but Cersei has hardly ever had much of an arc.