Hi Jeremy, I found the Clarkson Family Tree, trees.ancestry.com/tree/10640549/person/6071637348 which contains your Gustave Helmar Holmberg. Most larger libraries provide access to Ancestry.com. I thought you might want to make contact with the owner of this tree.
I think you've covered all of the possible sources for his teaching assignment in Granite County. Not every record was saved especially if he worked there only one year.
According to the Clarkson Family Tree in 1918 he enlisted in the U.S. Army at the time of his brother Oscar's death. He was discharged from the Army 11 Jan 1919 at Camp Greenleaf, Georgia. Successful Searching!