So, thanks to his eagle eyes and that census record found by Alan Boyd, I have found the UK (and Irish) end of Henrietta's story.
Barbara Jane Quinn (sometimes with Adeline thrown in) was Henrietta's oldest sister and James Alto was her brother. The family were Irish Roman Catholics and, on their mother's admission in court after Barbara Jane's death, sometimes their confirmation names were used. The Margaret on the 1871 record is Henrietta, so she was born around 1855 in Ireland.
The inquest into Barbara Jane's death was reported at length in the English press at the time and is summarised here, although there are some inaccuracies in this summary.
https://victorian-supersleuth.com/the-final-words-of-adeline-quinn/I have found further records of the father, Felix James, who is variously an engineer and a land surveyor in addition to Henrietta's marriage description of him as an architect.
Barbara Jane was killed (the verdict was manslaughter) on 13 June 1877. Henrietta married George on 23 Feb 1878 and gave birth in Boston on 4 November 1878.
I can't find her daughter's birth in those links, her daughter was Henrietta Mary, but will look again as I am not familiar with their use. i also still don''t know what happened to George Ketchley and Henrietta after November 1878.
Any help appreciated!