It did occur to me that Gilbert and Easton's backstory would have made a good novel in itself, but, indeed, probably more of a Broster novel than a Howells one :D I did like the moment where Gilbert goes back after their quarrel and finds the injured and unconscious Easton and knelt at the side of the man who was his friend again—but the hurt/comfort possibilities are certainly not developed fully.

Yes, those 'odd but harmless' attitudes are very interesting, especially when they go along with an explicit discussion of the tension between these friendships and heterosexual relationships. And it would have been easy to resolve that tension by (as Howells suggests is inevitable) having Mrs Farrell marry one of the men and ending their friendship, but the book's refusal to do something so simple (and I agree that the hinted Rachel/Gilbert isn't very plausible—a small concession to the expectations it's otherwise avoiding?) makes a much better ending.