It think we've uncovered an astonishing amount of information in a very short time, especially given the time period.
Who knows what further information could come to light if we just dig in the right spots.
Looking for information on his parents' deaths may help - what was Charles' standing in Kellie Castle?
He was no ordinary servant, or stablehand it seems.
Looking too at the previous Earl's affairs - Thomas Alexander Erskine was the elder brother of 'our' earl, and died in 1781 - could help too.
He held high office in Masonic circles, as well as musical talent.
I wonder about gravestones too? they can sometimes hold an wealth of information.
GenUk page on Carnbee parish
https://www.genuki.org.uk/big/sct/FIF/Carnbee
indicates there are various records held in various places that could eb worth following up.
Obituaries
Andrew Campbell has produced Fife Deaths from Newspapers 1822-1854 - a compilation of deaths recorded in local newspapers. Copies of this index are held by the Fife libraries and the Family History Societies.
looks promising if Charles did indeed die in 1827.