The family of Ker of Ferniehirst are mentioned in The Scots Peerage, Volume 5, under Ker, Lord Jedburgh, pp. 49-80, which can be found online here https://archive.org/stream/scotspeeragefoun05paul#page/80/mode/2up
There is a William Ker alias Kirkaldy, the son of Thomas Ker of Ferniehirst (they don't seem to have held the title of Baron) but he was married to Elizabeth Lyon, not Mary Ann Clifford, and there is no mention of a son Benjamin.
Based on that source, I would say that Benjamin Carr, isn't closely related to the Ker family of Ferniehirst, if he is related at all. There seem to be quite a few errors in what is currently on Wikitree in relation to this family, compared to Scots Peerage.