Hello Mr. Cornwell,
I have a guess to submit for Sharpe’s father. At first I thought he could be Dr Syn but I realised someone mentionned him already.
So my guess is.. could his father be Nicholas Tappitt aka Colonel Delacourt aka Black Nick?
He was in England around 1775. I find it interesting regarding your riddle that he was killed as he confessed for the crimes of Dr Syn, who was a smuggler himself. So he was taken for someome else (“you out, me in”). Regarding that riddle again.. a tapit is a kind of horse.
That would by very funny if I was right or at least close,
Best,
Imene