Fox12 said:
As for Cersei, she didn't kill Arryn, but she probably would have if given the opportunity, and if she thought he was a threat. Her eventual paranoia comes from her desire to protect her children, and eventually from a desire for personal power. It doesn't make sense for her to send her enemy where he can still be a possible threat. It would make more sense for her to keep him at the rock as a prisoner, where she could hold him prisoner if things went well, and kill him if they didn't. In any case, I like some of your insights.
Cersei's need to fear Eddard Stark died with Robert Baratheon, which is why she killed him (amongst other reasons, you can speculate Robert's death had been a goal of hers for some time). He was the primary threat to her safety. Once Eddard made the error of trusting Baelish, his status as a traitor was established, and any rumors he attempted to ignite regarding Cersei and the parentage of her children would've been easily dismissed as the ramblings of a traitor, much as they were with Stannis.
Eddard would've been little to no threat at the Wall. You could hypothesize that he could've betrayed his oaths and tried to rally the North, but the first honorbound Northman he came across would likely have taken his head off for deserting The Watch. Not that he ever would. Stark was very much a "my word is my bond" type, and he was paralyzed by the threat to Sansa in any case. She was the entire reason he gave his false confession to begin with. Stark was done. Cersei knew it.
Frankly, killing him at that stage was woefully reckless and kicked off a chain of calamities, both for the Lannisters and the realm. There's a reason Joffrey doesn't make anyone's short list for sensible, forward-looking rulers.
Cersei's paranoia doesn't really start to bloom until after Joffrey is dead. At that point she starts seeing schemers in every cupboard and comes completely unhinged.
Anyway, thanks for the collegial response.