I found a rundown of the Glendonwyn Baronage in an old book entitled "Baronage of Scotland" - Written in old english, but transcribed to internet, so somewhat more readable.
As indicated, William has been assigned as a son of Alexander, 9th Earl of Glendonwyn. This is a stretch to begin with, based on dates.
The more important fact, however, is that, while Alexander did have a son named William, according to "Baronage of Scotland" this William and his brother, Simon, both died young. Was William born to a later Glendonwyn earl?
10th - Robert: one son, who succeeded him.
11th - John: George, James, and William - all died without issue - and Robert..
12th - Robert: George, James, John, and Isabelle - all died without issue - and Agnes
13th - Agnes: One son, who succeeded her.
14th - Robert: Did have a son, William, BUT by the time he was born Charles was already in the U.S.

It is probable that Charles Clendening did, in fact, trace from the Glendonwyn family, but it had to be through a different branch which derived from an earlier son who did not accend to the family leadership. --- but who????

Alexander had a brother, William of Garquhinye, of whom the Glendonings of Mochrum in dominio de Parton, were descended. ... That would be a possibility.

Also, there is a record that, indeed, Mary Douglas was daughter of the 4th earl of Douglas and Margaret Stewart. She married the first Simon, rather than the second, though.

This information from the Glendonwyn Baronage history as recorded in "Baronage of Scotland"