Freedom of speech means not having your expression unlawfully interferes with.
Her expression has not been unlawfully interferred with, since absolutely nothing has prevented her from dissemminating her expression on numerous platforms of her choice. That is not what "unlawfully interfered with" means.
I believe Clanchy would have won a case against her publisher if they had insisted on her changing something that isn’t libellous, that they had already edited and agreed for publication.
No, she wouldn't. There's absolutely no way she would have won since they acted within the powers the terms that her publishing contract grant them. Publishing contracts have clauses governing exactly this kind of circumstance and publishing contracts are watertight.
It’s censorship
It is not censorship since Kate Clanchy had complete freedom of expression to choose from a wide range of options, many of which would have resulted in her book being published un-edited. The fact the option you assume she wanted, for the publisher to ignore her abusive behaviour and the resulting financial catastrophe, is not censorship.
Her publisher could have declined to reprint her book and returned her rights.
Which is exactly what they did, hence her publishing the book with another publisher.