You may or may not have any of the following information, I'm posting for archives and anyone "happening" to find it.
I found your family tree online while looking for additional information on Nelly. Her son, Richard, might have known something about Grandma...but he has passed per your tree. Her second marriage to Ross Culbertson sometime between 1920 and 1930. In the 1930 Census he was 47 (1882/1883), had a son Harry L., born about 1911/1912 in Nebraska. They were living in Ustick, Ada County, ID.
Idaho Death Index
Nellie Bly Culbertson, died in Nampa, Canyon County, ID, on 5 Dec 1950.
Ross died 1 May 1946 in Nampa.
Social Security Death Index:
Harry L. Culbertson, born 29 Jan 1913, "last residence" 12 Jan 1988 in Salem, Oregon. The "last residence" on the SSDI is NOT necessarily where a person actually lived, but is known only as the "home of record" by the Social Secuity Administration. I have found the deaths of persons far from where the "last residence was identified, and the person had never resided at that location. It is NOT necessary based on a benefit to the person, survivor or the lumpsum death payment; but, I have seen all. It is sometimes where notification of death was sent to the SSA; but not necessarily where the person had lived.
--The Oregon Death Index:
Harry L. Culbertson, died 12 Jan 1988 in Marion Co. While this somewhat confirms the SSDI, it is where he died, not where he had lived.
I didn't find a family tree for Harry. He might have known what became of "Grandma," but he had passed. His children, if any, may have heard a story, but unless they had done significant genealogy research as you have, they probably don't know. And, since there isn't a family tree, there probably isn't a person that might have further information.
Now, and last night, I looked at the 1910, 1920 and 1930 census' for ANY Adele, Del*, Blanch, in Idaho and Montana, using criteria of place of birth, place of birth of parents. I found no person meeting the basic criteria in those States. I would "speculate" the story of death might be true, especially with Nellie being old enough to "possibly" have been a part, or had lived through it.
So, information goes back to the limited potential records in the limited genealogy research area of Montana.
Good Luck. No need to respond to this post.