Ref “I cannot make out the rank of the officer in the picture. Perhaps an alert reader can puzzle it out.”
My two cents: The picture was probably taken when he was newly commissioned(most likely a young 2LT, possibly a 1LT depending on how quickly he got promoted). No campaign or service ribbons and the probability that it was a keepsake he kept on his weapon during the war, indicates it was taken before he went anywhere. -— Of course, there were a lot of folks in uniform who served out the war state-side.
Your analysis seems sound. As he was a young officer, it seems likely the uniform was near new; therefore he was in the service at very close to the beginning of the war, if not before Pearl Harbor.