US Civil War Highland Regiments

[wvpiper]

The Scottish ancestry was by that time fairly remote historically as most had been arriving in the US, and the South primarily, from 1700 or so on

small point, but the Ulster Scots/Scots Presbyterians/Scots-Irish primarily landed in Philadelphia, and migrated South, and West. See "The Scotch-Irish A Social History" by James Leyburn.
A few did land in the Carolinas - as did Highland Scots pre Civil War, but the majority came through Philadelphia.

Your major point is spot on, I would think.

As to the book mentioned, in the section of plates (i'm viewing it through Google Books, I can't see any actual plates, if they exist), the author also states:
The Union Light Infantry.......wore trews,or trousers, and a plaid, in Black Watch Tartan, that worn by the British 42nd; his fur bonnet is also based on that worn by the 42nd Highlanders.....This became Co F 1st SC Infantry until 1863 and thereafter Co C 27th SC infantry