I thinkxxyzzis absolutely correct in saying one of the problems with Clanchy's approach is she seems to have imagined her audience would be other middle class white people, and this feels like an extension of that. Other middle class white people (ie, society) were supposed to come along and - gently and kindly and with lots of validation - explore her prejudices with her, free from the intrusion of actual minorites, who weren't supposed to read the book or have feelings about it - it's all very "We're talking about you, not to you".
I think this is true, but if your book has a central theme of overcoming prejudice, surely it is reasonable to assume that you're mainly talking to people who might share those prejudices? Or should she have assumed that the victims of racism needed as much education about racism as the white middle classes? Can't imagine anyone finding that offensive. 
As to whether it is reasonable to sack someone for unintended controversy... I think her intentions are relevant. Firstly, on a personal level, because it is simply not proportional for someone to lose their livelihood and career (she may have a new publisher but she might not have got one) because of something she tried her utmost not to do, and explicitly asked the reader to overlook if in spite of everything she slipped up. And more widely, because if you condemn people to lose their jobs for inadvertent microaggressions you are telling them that getting involved at all is just one enormous gamble with everything they hold dear. It is a tactic designed to silence and it will work. I know an author who is currently rewriting her book to take out a rounded, complex Black character because she doesn't want to expose herself to being cancelled. Now, does white people including Black characters even touch the sides of racism? No, of course not. But does it help anyone if the characters in books get even less diverse? Also no. And if white writers think diversity is too dangerous to touch with a bargepole, publishers expect Black writers to fill the gap, and only ever write about being Black - witch is already a problem in publishing.
Basically I think it's quite a simple question: are we prepared to put aside anger in order to enable people to do better? Even if our anger is justified, even if we are so fucking tired of educating and persuading and manipulating that we could weep. Do we want to punish, or do we want to improve? If it's the latter, then we have to resist the urge to silence the truth - at all, ever.
If it's the former, and we can only bear to be with other entirely enlightened people, then we'd better make sure that either we have a Plan B, or we're the one in charge, because for as long as there's more than one person there we'll drop into the danger zone eventually.