Just an idle observation concerning the heroine of The House of Hidden Treasure: its heroine's name, Grace Dorrien, appears elsewhere in literature.
While Googling, I ran firstly into Tait's Edinburgh Magazine for 1856, which features a serialised story, Grace Dorrien: a tale. This rather wordy romance involves Grace, a pretty and pious young woman, who rejects a suitor, George Lambe, after her father goes unbalanced and takes against George following the death of Grace's little brother. At church, she meets and marries another man, Mr Keith. A year later, her father recovers his senses and wishes to apologise to George, but they go to his house and find he has pined to death, leaving Grace a guilt-tripping note. Grace realises that she loved him, and ends the story sadder and wiser. (The story seems in part a vehicle for the uncredited author's religious views, particularly in its hostile descriptions of the services and procedures in Independent Chapels).
Then there's the Grace Dorrien who appears in Duffield Osborne's The Wisdom of the Serpent, which appeared in Harper's Magazine, Volume CIII, No. 614, 1901. It's a rather nice story of US society social games, in which a society Queen Bee, Mrs Van Santvoord - who disdains ordinary social traditions as "the wisdom of the serpent" - attempts to engineer a match between Tom Kennicott and Mabel Strange. Mabel has refused Tom, so Mrs Van Santvoord advises him to court the flirty and inconstant Grace Dorrien to engage Mabel's jealousy. It goes entirely counter to plan, as Tom and Grace become a genuine item and marry happily. "The wisdom of the serpent" proves more powerful than Mrs Van Santvoord's schemes.
Given there's such a vast information space in English literature, these three Grace Dorriens may be coincidental, but it is such an unusual name that I'd love to know for sure if there was any conscious or subconscious borrowing of the name from the 1856 tale to Maxwell Gray's 1898 novel and Osborne's 1901 story.
Google Books finds, incidentally, a real-life Grace Dorrien in 1811, among subscribing members of the Society for Promoting Christian Knowledge. The surname, though very rare, did have its profile increased by Julia Kavanagh's 1875 novel John Dorrien.