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.