The narrator uses words like "presumably" because so many of the facts concerning the life and death of Jean-Baptiste (Almighty Voice) contradict each other. Even the descriptions of the picture of Almighty Voice contradict the picture itself. This is a story about the difficulty in interpreting history. Here we have a story about a Cree man (Native American), whose actual history has been obscured and manipulated by the descendants of his oppressors. Therefore, we can only presume that certain facts have been left out; and, we can only presume what Almighty Voice's actual history was really like. In presuming, we necessarily take part in recreating Almighty Voice's history and there is a responsibility in that.
Even the artifacts that still exist are difficult to precisely inspect. There is a piece of Almighty Voice's skull ("presumably" in a museum), but it is difficult to get a close inspection of it because it is in a glass case:
Precision is difficult since the glass showcase is at least thirteen inches deep and therefore the eye cannot be brought as close as the minute inspection of such a small, though certainly quite adequate, sample of skull would normally require.
The weapons used by the police are neatly kept but Almighty Voice's rifle has been neglected and appears to have mildew on it. All of these examples support the idea that history is written by the victors (of war, conquest, etc.) Almighty Voice's "voice" and life have been obscured by the victor's account of history. So, the narrator uses words like "presumably" and "perhaps" to underscore the contradictions of facts and the uncertainties of the general (victor's) historical record. The narrator uses phrases like "of course" to note the obvious contradictions and the obvious attempts to obscure the actual history/voice of Almighty Voice.