I am sorry I can't help you with the officer's account you have mislaid, but my curiousity was piqued by this story, and doubly so, since in Gardyne's "The Life of a Regiment.The History of the Gordon Highlanders 1794-1816," I can find no mention in the account of the retreat (pp.181-194) of the case of Serjeant-major Mackay. Perhaps I have an abridged version.
Your account of Lieut. Col. Napier ordering the court martial re-convened to alter the verdict is particularly interesting, since earlier in the book Gardyne describes in some detail how in the early years after the Regiment's formation, the officers of the Gordons were at great pains to avoid sentencing men to corporal punishment, indeed would go to great lengths to avoid punishing them at all, seeking rather to explain to them the error of their ways. This was particularly so under the command of Lieut Col Erskine, who was killed in Egypt in 1801. The abhorrence of corporal punishment among the Highland Regiments before 1800 was not confined to the Gordons, although their particular esprit de corps appears to have been remarkable on this count.
Gardyne's remarks on the death of Lieut Col Napier mortally wounded at Corunna are particularly illuminating in this regard: 'Strict without nagging, just without harshness, and considerate without weakness, he was a worthy successor to such an excellent commanding officer as Huntly an Esrkine. His best epitaph is in the words of one of his N.-C. officers, who described him as one “whom every man in the regiment adored, and to whom he was more like an affectionate father than a commanding officer.” ' (p.196)
Clearly, conditions on the retreat to Corunna were extreme and required extreme measures in the attempt to maintain order on the march, although, as Gardyne recounts, once the army turned to meet the French outside Corunna, morale and discipline improved miraculously with the prospect of battle. While the circumstances of the court martial and sentencing seem out of keeping with the culture of the 92nd, the fact that Sgt Major Mackay ended up retaining his appointment until the regiment returned to England seems less surprising.