AS you've been trying to locate Catherine's ancestry for a while you may have already searched for these
If the Catherine born in New York is correct her son Henry must be with a family member somewhere
Have you searched for a 5-6 year old as a lodger, visitor, relative other than son etc?
Remembering STEWARD, STEWARD, STUART, etc.
If you locate a good prospect the place of birth given would give you a location to search births to try and find a match.
Catherine thinking of "common decency" I would not expect to marry within months of being widowed(!); have you looked for a death of a Walter STEWART between 1865 and 1870?
Lastly a very long, and more difficult, shot is the movements of Lady Beckett could provide a clue to where Catherine joined her service