Have just been reviewing a new study of censorship in England, review forthcoming in due course in a scholarly journal, but it mentioned, as have previous studies, without really delving into, the role of publishing in gatekeeping. This work, and earlier studies, have emphasised the 'miasmatic' effect of the haphazard operations of the distributed nature of censorship in England from the mid-nineteenth century up to the later decades of the twentieth (the acquittal of Penguin books over Lady Chatterley's Lover did not usher in a free-for all) and that this led to a degree of caution amongst publishers as to what they were prepared to take on.
What has not, as far as I know, yet been undertaken is an exploration of publishers' archives (many of which are now available in various repositories) to see how far considerations of fears of difficulties over censorship led them to engage in dialogue with authors over what was and what was not prudent to express. (Before it came to the question of the difference between publishing in hardback versus a more generally available cheap paperback, a central issue in Regina vs Penguin Books.) There may also, perhaps, be a record of this kind of process among the papers of authors themselves. What were the compromises that were negotiated to enable publication to proceed?
I have also been given to wonder about the differing levels of what was permissable in different genres. In her memoir, You May Well Ask, Naomi Mitchison described the difficulties she had in publishing a novel of contemporary political and social life, We Have Been Warned (1934), in which she found that the freedom she had been accustomed to employ in her well-received historical novels in dealing with sexual mores met with entire horror. What had caused no problem when her characters wore 'wolfskins and togas' became potential matter for prosecution when they were clad in modern dress.
One also wonders whether the crime novel had certain liberties permitted.
On the more general question of who was an appropriate audience, I do not know if any work has been done on public libraries and either attempts to get potentially controversial literature onto the shelves for rate-payers' perusal, or protests by rate-payers that public money was being spent on noxious matter. A very anecdotal hint discovered when pursuing reception of the first Kinsey Report suggests some libraries had a system of certain tendentious items being kept 'in the Librarian's office' and only produced on request, with their approval.
Historical research, while answering some questions, will always tend to open up further lines of enquiry!