A reader and fellow Elsie Houston fan has sent me some clippings from the New York Herald Tribune that shed light on some matters EH.
First, the Tribune obit (unlike the NYT) hints at the contents of the suicide notes: "she was upset about financial matters."
Second, her father was indeed James Frank Houston (who was apparently still alive and practicing dentistry into the 1940s) whereas her mother was "a Brazilian whose family emigrated to Rio de Janeiro from Portugal 300 years ago."
Third, a long profile by William G. King in the New York Sun (March 30, 1940) repeats a claim I've read elsewhere that she sang in the first Paris performance of Manuel de Falla's "El Retablo de Maese Pedro." If this is true, she would have been 20 years old and still a student of Lilli Lehman. It is more likely she sang at a later performance while she was a student of Ninon Vallin (a Falla favorite).

I'm heading out to the Boston Public Library tonight to see what else I can glean from the other New York papers.