Thank you for answering! And ooh split northern lords will be interesting to see in the next book~ Also in the books, Do you think Tywin ever considered it might be dangerous for Cersei to marry & have children with a different man? What if she did and the child was born with dark hair? Wouldn't that make the people even more likely to see her children as bastards considering so many in Robert's family have dark hair/blue eyes? You'd think he'd want to marry her to a blond man, just in case.
I don’t think it works that way? Sure, the Baratheons have Super Dominant Fantasy Genes. But not all dark-haired people in Westeros have the Baratheons’ Super Genes. For example, Jeyne Marbrand is shown to have dark hair in official artwork, but all of her children were Lannister blond. And there’s an awful lot of blond Hills running around Terros too. And Rohanne Webber was a redhead, but it seems her children were all blond. So the Lannisters must have some Strong Fantasy Genes as well.
So if Cersei had a child by, say, dark-haired Willas Tyrell, it might be blond or it might be dark-haired. If it’s blond, then the Lannisters can hold it up as “proof” that Cersei’s children are all legitimate. If the new baby is a brunette, then it just becomes the child Tyrion theorized about, the “odd man out” of Lannister phenotypes. After the birth of a brunette child, people might simply conclude that Cersei’s kids have a 75% chance of coming out blond, 25% chance of other hair colors.
Hair color alone wasn’t enough to prove Cersei’s adultery; there were other factors that led Ned (and Jon Arryn, and others) to discover her treasons.
But all this talk about Lannister phenotypes is moot, because the horse has already left the barn. When Tywin says, “You will marry and you will breed. Every child you birth makes Stannis more a liar,” he’s delusional. Almost everyone in Westeros suspects that Joffrey, Myrcella, and Tommen are bastards. I think Tywin is the only man on Terros who won’t admit this to himself, like, I am firmly in the camp that Tywin has willed himself into believing that his grandchildren are legit, and he absolutely refuses to even see all the evidence to the contrary. But Tywin has blinders on about a lot of things, and there’s nothing Cersei can do to change public opinion at this point. I mean, they’re literally screaming about the Lannisters’ illegitimate claim from Dorne to the Wall.
So, Tywin’s marriage plan is illogical, but I don’t think it’s illogical because of the potential for dark-haired children. It’s illogical because it places an impossible burden on Cersei; it’s impossible to convince everyone that Stannis is a liar at this point, regardless of the hair color of Cersei’s fourth child.
At best, we might euphemistically describe Tywin’s plans for Cersei as wishful thinking on his part as he plots to get Cersei out of the way (because Tywin is a misogynist), but this “you will breed” idea of his is like using a teacup to bail water on the Titanic. “No matter what we do, House Lannister will founder. It’s a mathematical certainty.”