I know McKendra is an unusual surname and the surname we use today has been altered slightly. It took me quite a few years of trying to trace my ancestors before I realised they were using a slightly different surname. My ancestors were literate and that's how my g g grandfather signed his name on a document I have from around 1840.
The idea of a single “correct” spelling is very much a more modern concept and from my experience of Irish research, spelling will vary from record to record, especially so with native Irish names where, in addition to varying the spelling, the O & Mc prefixes can be added or removed. There were various reasons for all this, illiteracy, indifference and differing officials spelling in their own individual ways. Even literate people varied the spelling.
In his Special Report on Surnames in Ireland, published in 1909, Mathieson includes numerous examples of variation in the spelling of surnames which depended on the whim of the holder. “Some years ago the marriages of a brother and sister in the same family were solemnized in a Registrar’s office. The son gave his surname as “Faulkner” and his father’s name as “Faulkner.” The daughter gave her surname as Falconer, and her father’s surname as Falconer. Both marriages were subsequently re-solemnised in a place of worship, and the same orthographical differences were found to exist in the records kept by the officiating Minister.”
I do think the advice about keeping an open mind on the spelling of this name is sound.