Hi Julie,
I'm not sure I can offer much direct help, but the name Catherine Galloway ( my 6 x great grandmother) and that of the quite unusual name Montgomery (a girls name in my ancestral lineage and boys middle name in your cited case) and the era in Lochmaben are surely too many matches or similarities to be coincidence...
My 6 x great grandmother Catherine Galloway is not your Catherine as she was a daughter to James Galloway and Fergusia Montgomery. My Catherine was born 1757 and later married my 6 x ggf James Green. Given the names you cite as the parents of your Catherine, and the surnames of my 7 x ggf and ggm Galloway and Montgomery - does this explain where your William's name derives
It is entirely possible that your William Montgomery Galloway is a sibling to my 6 x ggf Catherine Galloway.
As far as emigration records are concerned I do not think Ancestry helps much as the information you provides indicates that she emigrated circa 1854/5, at least I cannot trace her on the immigration lists...
I haven't tried looking or researching to see even if such records are held locally in Dumfriesshire ( to account for immigration passages etc), but it may prove useful for you to make email contact with the Ewart Library and its archivists to ascertain if they hold anything along these lines...
The Ewart Library is a fine old sandstone building in Dumfries and details can be viewed online here:-
http://www.burnsscotland.com/venues/interactive-map/ewart-library.aspxPlease do let me know if any of the above strikes any bells or possible links...
Kind Regards
Kenny