IMHO William Edmonstone, Lord Edmonstone of Culloden is the son of:
Sir John "Baron Boyne" Edmonstone, de Edmundiston (1325 Cambusnethan, Lanarkshire, Scotland. - 7 APR 1410 Cambusnethan, Lanarkshire, Scotland.) & Lady Isabella Eupheme (nee Stewart) Edmonstone (1348 Dundonald, South Ayrshire, Scotland. – 1410 Douglas, South Lanarkshire, Scotland) https://www.findagrave.com/memorial/154431417/isabella-eupheme-bruce ~ She was the Daughter of: King of Scotland Robert II Stewart (1316 Paisley, Renfrewshire, Scotland. – 1390 Scone, Perth and Kinross, Scotland.) https://en.wikipedia.org/wiki/Robert_II_of_Scotland & https://www.findagrave.com/memorial/9457579/robert-king_of_scots
There are some serious issues here with the royal and high noble Scottish lines. The following profiles are duplicate daughters of King Robert II. And both are tied through duplicate paths to various MPs.
Medlands states that Isabel was Robert II's daughter by his first wife, Elizabeth Mure, while this book more specifically about the Edmonston family has her as the daughter of his second wife, Eufemia Ross, relict of Randolf, Earl of Moray. One could spend a month working on corrections for this line and still get nowhere due to the many locked MPs involved.
https://digital.nls.uk/histories-of-scottish-families/archive/95353...
https://fmg.ac/Projects/MedLands/SCOTLAND.htm#_Toc65391153
Euphemia de Ross, Queen Consort of Scotland
However, my source also apparently asserts that this William Edmonstone was really the son of Archibald (not John). And that Archibald was likely the younger brother of John Edmonstone.
https://digital.nls.uk/histories-of-scottish-families/archive/95353...
Assuming I picked up the right ones (spouses of the Isabella duplicates), these would be the duplicate John Edmonstones.
Sir John Edmonstone of that Ilk and Ednam