This testimony is partially validated by an item reported by Howard V. Jones:
From GWCE's testimony  we know the following participants at King's Mountain, and their fates:
From the above we see that Major Edmiston lost
as well as having his nephew person:Robert Edmondson (6) wounded. All of these men served together in Campbell's unit under the command of Major Edmiston. Of the seven members of the extended Edmiston family, three, Major William Edmiston, his brother Samuel, and his brotherinlaw Captain Davied Beattie, were unwounded.
Some of the relationships Edmiston described are shown in the following diagram:
Note that David Beatie and Captain William Edmiston, are not identified on this diagram. These two individuals each present an interesting problem in Edmiston family history. Since GWCE identifies them as his uncles by marriage, they are presumably the husbands of the of two daughters of John Edmiston: Mary and Elizabeth. David Beattie, who is documented by Source:Moss, 1990 as serving at King's Mountain, is in fact shown as having a wife "Mary". However, Moss indicates she is a cousin, and her maiden name is "Mary Beattie". This seems to contradict GWCE's testimony. Checking Ancestry there are a goodly number of persons tracing their descent from David and Mary, but they consistently give her maiden name as Beattie. In this they may simply be following Moss, or it may be that there is some specific documentation which shows her to be a "Beattie" rather than and Edmondson. (In which case GWCE confused the family relationships.) Perhaps close examination of Moss' sources would shed some light on this problem. Overall, it would seem that GWCE's testimony would carry considerable weight. Since he is a contemporary of the events, and should have been in a position to know the family relationships, it seems more likely that David Beattie's wife was Mary Edmiston, not Mary Beattie.
Somewhat more problematical is the identification of William Edmiston as an uncle by marriage. If David Beattie's wife is Mary Edmiston, then William Edmiston's wife is presumably the remaining daughter Elizabeth. At issue though, is the identity of the parents of her husband William Edmiston. This was discussed in an article by John Rison Jones from Vol 14(4) of the "Edmondson Family Association Bulletin". Jones is quoted as saying in that article:
Moss, 1990:76 tells us that Captain William (not to be confused with Maj. William Edmondson", later known as Col. William Edmiston) "and two of his brothers" were killed at King's Mountain. The other Edmiston's killed that day were Robert and Samuel; they are otherwise identified as the brothers of Col. William Edmiston, so could not be the brothers of Captain William Edmiston. Captain William would, however, have been Col. Edmiston's brotherinlaw. 
Who then, is Captain William Edmondson, and how is he related to Major William Edmondson (other than through his marriage to Maj. Edmondson's sister Elizabeth? One possibility is that he is Person:William Edmiston (3) son of Person:Robert Edmiston (2). That would make him a cousin of Major William Edmiston. That seems generally consistent with the known facts, with the exception of the fact that we know that Person:William Edmiston (3) survived until 1787 when he wrote his will. Its possible that he was was fatally wounded at King's Mountain, but did not succumb to his wounds until 1787/88. That would explain how GWCE could describing as being killed at King's Mountain, but his will not written until 1787. That COULD be the explanation, but that is only speculative. On the other hand, there's evidence in Washington County of a William Edmiston dying in the fall of 1780 or early winter of 1781. In a court record for for 15 May 1781 a "William Edmiston, deceased" is replaced as the administrator of the estate of one Robert Reagh. It seems likely that the William Edmiston mentioned in this record is in fact the one killed at King's Mountain the previous October.
The following land record also seems relevant to this quesiton:
This shows a William Edmondson deceased by August 1781. Again, this would seem to be a reference to Captain William Edmondson who died at King's Mountain, and precludes the possibility that he lived on for several years dying of his wounds c1788/89. Therefore, he is not William (3).
So, if Captain William Edmondson can not be William (3), who is he? Another possibility is that he is descended from Person:William Edmondson (16), brother of John (2) and Robert (2) who settled on Borden's grant about 1740. WIlliam 16 did have a son William who could be of about the right age to be William (17). Whether he is or not remains to be determined. Its possible, that William (17) is in fact a grandson of William 16, either through son William, John, or Robert who are identified in William (16)'s will.