Thank you for your very exhaustive response. I agree with some of your points and disagree with others. I hope you don't mind that I only limit this response to one of your points:
She presents this wife of one of her white characters as a very normal person. Dementia doesn't select it's victims based on skin color. Suffering from dementia also does not turn the person into a freak.
But perhaps you could explain what Shriver's problem is when she decided to have one part of an interracial marriage suffer from dementia?
I must use the caveat that I haven't read the book, so my knowledge is only second-hand. But from what I understand, the issue is not that she has dementia--there's nothing wrong with a character having a mental disorder--but rather that her entire purpose in the book is to embody a white male character's comeuppance. Her decline, disability, and humiliation are never presented to share anything meaningful about her, but to embarrass and degrade another character for his poor decisions. Seems kinda shitty for her to only exist as a plot device in service of a white character's arc.
The critic whom Shriver alluded to for calling her racist took the time to respond, and I'll quote the salient pieces of that response:
Shriver’s 12th novel is set in a near-future American dystopia where many of the concerns currently expressed by conservatives finally have been realized. After an immigration amnesty, the country is flooded with “Lats” who elect a Mexican-born president who presides over a devastating economic collapse, in part created by runaway entitlements. Shriver observes President Alvarado’s “baby-faced softness only emphasized by the palatalized consonants of a Mexican accent,” a stereotypical image of a pudgy, lisping Mexican that links his perfidy to his ethnicity as would an elliptically described hooked nose on a loathsome Jewish character.
The two black characters are similarly ill-treated. One, a social worker, is the novel’s only character who speaks sub-standard English. After Alvarado renounces the national debt, she says, “I don’t see why the gubment ever pay anything back. Pass a law say, ‘We don’t got to.’ ” It was once common in newspapers, fiction and nonfiction to report the speech of “ordinary” people in standard English, while voicing minorities in dialect or vernacular, as they might sound to white ears; this still happens from time to time, unfortunately. By recording only the speech of minority characters in sub-standard English, you stigmatize the entire ethnic group as something other than normal. No one speaks perfectly. Respect for your characters suggests that if you record one’s solecisms, dropped consonants, drawl or brogue, you will faithfully record everybody else’s, too.
The most problematic of Shriver’s minority characters is an African American woman who has married into the white family at the heart of the novel. She suffers from early-onset dementia and is a danger to herself and to others. As the economy collapses, the family loses its home and treks across Brooklyn with the woman at the end of a leash. A plot development that features an uncontrollable black person who has to be kept under restraint like a dog seems guaranteed to hurt and provoke outrage. I wrote, “If ‘The Mandibles’ is ever made into a film, my suggestion is that this image not be employed for the movie poster.” I was thinking of ads in bus shelters and, honestly, I imagined they’d be wrecked.
I don't necessarily take this to assume that Shriver is racist. The actions of characters in a work of fiction obviously don't have to be a representation of the author's politics. Authors can include racist elements in order to critique them or highlight the illogic and hurtfulness of racism.
But I think it's telling that Shriver chose not to engage with any of the criticism on that basis. She just got upset that anyone might suggest she had racist portrayals in her book. As the saying goes, hit dogs holler. She could have engaged meaningfully with such criticism and explained her motives better. Instead, she took umbrage and used her platform to rail against critics and letter writers who dared to suggest she might have a been a bit insensitive. It's not like she was receiving death threats or otherwise being treated viciously. She was getting fairly run of the mill criticism, as any writer should expect.