The question has been asked, why would Captain Vale feel compelled to challenge the Confederate version unless he knew it to be false?
This assumes that Captain Vale read a Confederate account or exchanged correspondence with Confederate veterans about this incident. Having read most of "Minty and the Cavalry", I find no evidence that he did so.
Vale's intended audience was "National". That means he wrote for U.S. veterans, their families and loyal supporters of the Federal cause. Whatever ex-Confederates might have to contribute towards a history of "The War of the Rebellion" was of no relevance to him.
Early in his book Captain Vale condemns all Confederates as traitors worthy only of a traitor's fate. He later dismisses Confederate versions of the war as "cock and bull stories" invented to explain their many defeats. With this sole exception, Vale doesn't trouble himself to explain what the rebels might have claimed.
Why bother in this one case? As already mentioned, Vale assumes that readers will agree that all Confederates are traitors, their versions of the war being mere fabrications invented to draw attention away from utter defeat on the battlefield. Anyone outside the "National" audience has been dismissed, so we can only assume that stories about Captain Freeman's murder had been circulating among Northern veterans in the 1880s.
Vale concerned himself with only with Federal comrades in arms, Confederate "cock and bull stories" being unworthy of his attention. The only version of Freeman's death he would have felt compelled to challenge would have been one believed by Northern veterans.
Anyone have a different conclusion?