Clearly these are two different persons named Una McIntosh. Scott Waldo McIntosh is describing a brother of Col William McIntosh who would be the uncle of Col D.N. McIntosh. Scott McIntosh and his father, also named William, are African-American but Una is Creek. I didn't find an Una McIntosh in either the 1st or 2nd IHG but he may have used his Creek name and there are several persons whose names began with what phonetically is like Una (e.g., Oo-na). Scott Waldo McIntosh assumes that Una was killed at the battle of Honey Springs because "he never returned".

Both Phillips and Maxey refer to D.N. McIntosh as Una -- Phillips in his letter to Una McIntosh and Maxey referring to that letter. It seems likely that D.N. may have been named after his uncle, assuming Scott Waldo McIntosh is remembering the story and relationships correctly.