That's what doesn't work for me and what convinces me this is just a funny coincidence (not all that funny to me, I don,t think it fits closely enough to be intentional, but I see why you find it funny). I have a really hard time with RJ making a joke about Graendal being useless to the story. That's not how he thought about his characters, and I'm convinced he would have disagreed with the notion of putting it that way. Graendal didn't die because she had become "useless". She died because her purpose in the story was to die that way, precisely at that point of the story, when Rand had reached that precise state of mind. RJ didn't waste her because he had no more for her to do, he built up to that death. Specialist of the human mind, she died when Rand was at the height of his mental problems, and she died because in her overconfidence she thought she had him figured out and he made a move she would never have expected. She thought he was blocked about women, and she thought he was too soft to destroy so many innocents just to kill her. She must have thought he was unwisely making his opening moves in the hope to chase her away with mind games, and she was surely confident she would outwit him at that game. She didn't see the "dark mad Rand" he had become, and she died for it. She's supposed to heal the mentally ill, and instead she puts her talents to driving people crazy. She got killed by a madman out of control... how fitting.

Before he got there, all her plotting in Arad Doman drove him crazy. Graendal ruined all his plans. She will never know, but she has won. The situation looked so hopeless he totally gave up and left - but he passed his frustration by killing her before leaving. Had she survived, she could have gone to Moridin and get a "good job, Graendal" from him. He didn't manage to bring any sort of order to AD, everytime he thought he was on the right path, something went wrong (for a long time he thought he would manage to get a King elected, and Graendal let him think he could.. but after she let him get his hands on Chadmar - making him believe his plans stood a chance of working - she had her removed, and she killed the councilors he had found etc. She was frustrating his efforts in an annoying way, as ordered, making Rand run in circles). She didn't have to do anything to make his efforts with the Seanchan fail, because they fell on their own. Graendal has done her job - she was far from "useless" in Arad Doman, really. You can even say Jordan killed her because otherwise she would have been far from useless once Rand went away.

Moridin didn't fool Graendal, she only has herself to blame for her death. And this doesn't have anything to do with Aran'gar either. So beside the fact she died, it has not much to do with the comments of Aran'gar in KOD (especially that Aran'gar didn't even reflected that she would kill Graendal once she was useless - we just surmise that she probably would have given the chance, so technically that remark wasn't even about Graendal).