Oh yeah, I agree, but if you're looking at it as a mother would look at it. Tommen killed himself...why? As a direct result of her actions.

Joffrey was murdered by Olenna why? Because Cersei let Olenna into her house.

Myrcella was murdered in revenge for the death of Oberyn who died why? Because she made the error in charging Tyrion with Joffrey's death.

Tyrion sending Myrcella to Dorne doesn't guarantee her death...killing Oberyn does. Myrcella was perfectly safe in Dorne until the death of Oberyn. They're not killing Myrcella because the mountain raped Elia etc. either. They killed Myrcella because Oberyn died.

If Oberyn had found the Mountain in the streets and fought him there and died the Sand Snakes might have killed Myrcella in revenge. And if that had happened no blame could fall on Cersei. But that's not how it happened. Cersei wrongfully accused Tyrion and that caused a trial and that trial is what led to the deatrh of Oberyn, which is what caused the need for revenge which is what caused Myrcella's death.