The young woman who returned from Hawaii to San Francisco in 1866 on the
same ship as Mark Twain was Isabella A. Cotton. In December 1866, just
before leaving San Francisco for New York, he wrote her two letters (or
rather, one letter and a brief supplementary note) which are printed in
_Mark Twain's Letters, Vol. 1, 1853-1866_, pp. 371-72. A footnote that
begins, "Little is known of Isabella A. Cotton...." reports that she
lived, at least briefly, with the family of the keeper of the Fort Point
lighthouse in San Francisco.

This may be a little more than you had before, but it certainly falls in
the category of "no comprehensive answers" that prompted your question in
the first place!

I haven't found any source on G. Armstrong other than Frear's _Mark Twain
and Hawaii_, to which you refer in your query.