I have also had occasion to look at the Dora Marsden files relating to The Freewoman among her papers in the library at Princeton University, which the latest biographer does not appear to have consulted.
The standard narrative for the reason for Cicely Fairfield's switch to the pseudonym of Rebecca West, and the one which features in the Prologue to the new biography, has tended to follow her 1926 essay in Time and Tide and to state or at least imply that she chose to publish under a pseudonym because The Freewoman was considered so scandalous that her family (in some accounts, specifically her mother) refused to have it in the house. It is, of course, possible that this version is substantiated by correspondence I have not personally seen in one of the several collections of West papers.
However, according to an early letter to Dora Marsden about writing for The Freewoman, over the signature of Cicely Fairfield, she wrote:
I should like to reply to Lady Mayer's letter, but I cannot do it over my own signature. She is a power on the L.C.C. [London County Council] and it might conceivably happen that my sister [the doctor Letitia Fairfield, employed in the Public Health Department of the LCC] would be sacked for my heresies. If you will allow me to answer it over a pseudonym I will send something in by Monday morning at latest.This suggests rather different motivations, and a concern for her sister's career that might not have been anticipated given their fraught relationship and much-recorded fallings-out.
It's possible to wonder if she improved the narrative in later telling - the idea of a young woman writing hard-hitting journalism in a shockingly radical feminist publication barred from the house by her mother makes a much better story - or whether, looking back after a dozen years of significant personal turmoil, she simply misremembered and conflated her mother's disapproval of the journal with her decision to take a nom de plume. It's also possible that the reason given to Marsden could have been a face-saving excuse.