I, too, just noted the "imbecile" note in 1911 census for Harriet Fisher. Listed as a scholar when living with father in 1881.
Apologies if I am missing something but, given she married, why would she not be a Harriet Hall?
As far as I know, in the UK (as in Australia) it has not ever been against the law to use whatever name you want, so long as you are not using a name different from your birth name (or your husband's surname) for the purpose of deceiving or otherwise committing a crime. If she has left her husband, or has been deserted by her husband, then to all intents and purposes the marriage is over, even if no formal declaration of a civil court has finalised any divorce.
I think you will find that "Mrs" as a title in any decade of the 19thC could have a broader meaning than say our mid 20th century families would have understood or expected ....
There's actually plenty of examples of females in New South Wales (Aust) in 19th Century who continued to use the surname they arrived with, long after marrying in NSW. While some of those females were transported under sentence of a civil court, some were simply transported as part of the household of the officers of various regiments sent as garrison forces (UK garrison forces in NSW to about 1870s, some came as wives of men chasing their fortunes in the gold rushes, some came as Orphaned girls, and the various immigration schemes can provide many other reasons ...
Trying not to get too far off the track, but females could live under their birth surname in the 19thC, and it was not something 'new' to the Feminist Movement of the 1970s

.....
Obviously I cannot speak about UK records in the 19thC, although I do have a fair working knowledge of NSW official record keeping (or lack thereof) in the 19thC (and it varied widely through each decade ! ) and NSW was of course just one of the British Colonies in the Antipodes
Question of the Aussie RChatters reading this thread ..... Anyone searched Trove or Papers Past for any sightings of colonial reports of this dastardly horrible man .... that giblet has so carefully typed up for Carol and us all.
Cheers, JM