Hi
From what I can see, your Mary's background is most interesting. If her father was a 'surgeon of any standing' I would have expected her origins to have been in a major Sussex town, rather than minor one. Brighton, Lewes, Horsham, East Grinstead and Chichester come to mind. Gut feeling however draws me towards two Sussex rural villages called Catsfield and Crowhurst just outside Hastings, neither of which I would have expected a practising Surgeon to live. Hence my thoughts that he could have been with the army or navy and perhaps some conflict or plague resulted in Mary being orphaned and being placed in Battle Workhouse that served that area.
An alternative suggestion comes from the dual use of names, for there is also a possibility that she was the base born child of a Rugby Chisholm & Hannah Ransom for your research indicates she had no family about her for support. Lastly have you ever considered that she was just an over romantic juvenile runaway?
I reccomend you have a look on Genes Reunited for the several people who have an interest in the Mary Ransoms (various spellings) from Catsfield or Crowhurst also born around 1825, or try to find them on the 1841 Sussex censuses. If nothing else, that would help eliminate those particular Marys from your field of research.
Roy G