I found her. I am so thankful to Family Search for indexing all of Michigan's death certificates from 1921 to 1952. I am so proud that I was one of the people who helped index them! I found Mary Herr's death record and her name had changed from Jamieson to Meyers, though I can't for the life of me find any record of her marrying a second time, or find her living with anyone named Meyers in a census. I found her in the 1940 Census with her two youngest children, Grace and Betty Ann Jamieson. I can't find where she and George W Jamieson divorced, and he didn't die until 1964. I think.
But I know it's her in the death record because it lists her parents as Frank Herr and Christine Knolty. I'm so happy to have found her! I wish I knew where she was buried, and I wish I could know what happened to her daughters, who were 12 and 16 in the 1940 census, after she died. I won't be alive when the 1950 census is released, but I suppose by the time it is, I'll have all the answers to every one of my questions. Or at least I like to think that all questions are answered in Heaven.