Could the technologies needed for a double-row radial engine have been invented in the late 1910s or early 1920s?
Short answerThe mounting and hub are the difficult parts, making the early 1930s more plausible than the 1910s or early 1920s after the necessary incremental development.
NotesA double-row radial is not created simply by placing more cylinders behind the first row. The hub, mounting, cooling, and mechanical relationships have to work together reliably. That integration requires substantial incremental development. The result is a plausible acceleration into the early 1930s, but not an effortless leap back into the First World War era.