What plants/flowers would have been historically planted in the midwest at the turn of the century?

Most dubiously there would have been Hollyhocks, Cosmos, Roses, Salpiglossis, Zinnias, Heliotrope, Phlox, Delphinium, Lupines; also occasionally unusual exotics such equally Canna & Eremurus.

Victorian & English gardens were still very popular into the 1920's, so many of the particular
plants may have been in your ingenious garden scheme.