Lt Burns is obviously over 50 years old (the actor is 51 during filming), while the army's promotion ladder was glacially slow after the Civil War, the man would have been separated from the army long before then. If for nothing else, to make a billet for a younger officer to get combat experience on the frontier.
Hale uses binoculars to view the approaching Comanches. Binoculars were not commercially available in the United States until the late 1800s/early 1900s.
Barney didn't get involved when the Indians attacked. He was seen earlier in the evening, sleeping. Then he isn't seen again until the end of the story as the wagons are moving out.