also not sure if catelyn or margaery

Cersei's prophecy ask

I have just sent this Ask to joannalannister, but since opinions on this subject interest me, I’m copying it here. If anybody wants to discuss, I’d be happy to hear your take :D

Hi, I have perused a bit through your Cersei tag and haven’t found something about this, but if there is, please forgive me.

So, one of the things I’m wondering about are Maggy the frog’s prophecies. My doubts about it are several. I’ve read somewhere (probably on Tower of the Hand) someone who was disappointed by the introduction of Cersei’s prophecies so late in the series, and lamented that it changed their perception of Cersei (the sense was that it weakened her strong-willed character to have her acting based on a prophecy.) This comment made me reflect.

First of all, do you think that Cersei’s prophecy was something that Martin decided from the very beginning? For example, I admit not having re-read the books after AFfC, so I may totally be wrong, but Cersei’s behaviour toward Sansa don’t seem to me consistenty dictated by the “younger and more beautiful queen” threat. Sure, Cersei has Lady killed and is certainly not Catelyn Stark-motherlike toward Sansa, but she does give her counsel (albeit twisted) and in general doesn’t sound as if she wants truly to destroy Sansa as she later does with Margaery.

So (second question), was Cersei’s life as dictated by the prophecy before Joffrey’s death as she was after it? Was Joffrey’s death the trigger that reminded her completely of Maggy the Frog’s words? She also wasn’t completely horrible to Tyrion before that; I remember a scene in which Cersei dances happily with Tyrion at the news of Stannis killing (or marching against?) Renly.

Sorry for the lenghty message and thank you!