SPOILER: Who Else Disputes Tom's Version?
I thought Tom's story of shooting Valance was to "save the day" and ensure Ranse would accept the nomination and go to DC. THAT was his sacrifice.
The flashback makes me want to echo Annie in "Misery:" "Have you all got amnesia?" and now paraphrasing her: "There weren't two cock-a-doody shots!" Also, in the original scene Tom arrived late, which I took as his real, spontaneous words of the moment.
I think Ranse did shoot Valance - he was close enough to not need much marksmanship.
Seeing it that way makes the story fit together better for me.