Steve, I appreciate your comments and your experience and always have a good deal of respect for a healthy skepticism. I, too, suspect that it was very hard for most "collectors" to resist the temptation to "polish and improve", whether they were in the 16th century or the 20th century. I doubt whether the "improved versions" went back into the oral tradition [except perhaps in the 20th century when a collector might have been a performer as well]. They are probably what got printed for the scholars' frustration. At this point I am primarily interested in any hints about something being actually sung in the 18th century. I am more focused on the fact that a certain ballad was mentioned as "coming from the singing of so and so" than on the actual text of that ballad. I am looking for "living proof" so to speak. I do recognize that even with this, we have only the "collector's word".
I have gone back over my 18th century survey from Child and pulled out all of the references that I can see to something that looks like "oral tradition". However, I quickly discovered that many if not most of them referred to "recitations". Does this always mean that they were "spoken" as opposed to being sung? Here is what I found, other than Herd, Percy, & Mrs. Brown, to start with:
BURD ELLEN AND YOUNG TAMLANE
Maidment's North Countrie Garland, 1824, p. 21. Communicated by R. Pitcairn, "from the recitation of a female relative, who had heard it frequently sung in her childhood," about sixty years before the above date. 
CAPTAIN WEDDERBURN'S COURTSHIP—B
Kinloch MSS, I, 83, from Mary Barr's recitation. b. Lord Roslin's Daughter's Garland. c. Buchan's MSS, II, 34. d. Jamieson's Popular Ballads, II, 159. e. Harris MS., fol. 19 b, No 14, from Mrs Harris's recitation. f. Notes and Queries, 2d S., IV, 170, "as sung among the peasantry of the Mearns," 1857. [from Child: "Jamieson writes to the Scots Magazine, 1803, p. 701: " Of this ballad I have got one whle copy and part of another, and I remember a good deal of it as I have heard it sung in Morayshire when I was a child."]
SIR PATRICK SPENS—H
Minstrelsy of the Scottish Border, III, 64, ed. 1803; I, 299, ed. 1833; "taken from two MS. copies, collated with several verses recited by the editor's friend, Robert Hamilton, Esq., Advocate." [Child says: "...H, was made up from two versions, the better of which was G, and five stanzas, 16-20, recited by Mr Hamilton, sheriff of Lanarkshire. Mr Hamilton is said to have got his fragment "from an old nurse, a retainer of the Gilkerscleugh family," when himself a boy, about the middle of the last century."]
Minstrelsy of the Scottish Border, III, 51, 1803. Communicated to Scott by K. Williamson Burnet, of Monboddo, as written down from the recitation of an old woman, long in the service of the Arbuthnot family.
Minstrelsy of the Scottish Border, II, 102, 1802, chiefly from the recitation of an old woman residing near Kirkhill, in West Lothian.
WILLIE AND LADY MAISRY—A
Motherwell's MS., p. 498; Motherwell's Minstrelsy, p. 370. From the recitation of Mrs Notman, then far advanced in years, with whose grandmother it was a favorite: September 9, 1826.
THE WIFE OF USHER'S WELL—A
Minstrelsy of the Scottish Border, II, 111, 1802, from the recitation of an old woman residing near Kirkhill, in West Lothian.
Motherwell's MS., p. 480, from the recitation of Widow Michael, a very old woman, as learned by her in Banffshire seventy years before. August, 1826. [Child's notes]
Motherwell's MS., p. 165; Motherwell's Minstrelsy, p. 269. From the recitation of Mrs Thomson, Kilbarchan, seventy years of age, as learned from her mother at the Water of Leven, Dumbarton, when she was ten years old. March, 1825. [Child's notes]
THE KNIGHT AND SHEPHERD'S DAUGHTER—K
Motherwell's MS., p. 226. From the recitation of Widow McCormick, Westbrae, Paisley, 1825; learned of an old woman in Dumbarton, thirty or forty years before.
KING HENRY FIFTH'S CONQUEST OF FRANCE
a-d, broadsides. a. Among Percy's papers. b. Roxburghe Ballads, III, 358. c. Jewitt's Ballads and Songs of Derbyshire, p. 1. d. Chetham's Library, Manchester, in Hales and Furnivall, Percy's Folio MS., II, 597. e. Percy papers, "taken down from memory." f. Nicolas, History of the Battle of Agincourt, 1832, Appendix, p. 78, from the recitation of a very aged person. g. The same, p. 80, source not mentioned. h. Tyler, Henry of Monmouth, II, 197, apparently from memory. i. Percy Society, XVII, Dixon, Ancient Poems, etc., p. 52, from singing. j. Skene MS., p. 42. k. Macmath MS., p. 27, from tradition. 1, m. Buchan's MSS, I, 176, II, 124, probably broadside or stall copies.
JOCK O THE SIDE—B
a. Caw's Poetical Museum, 1784, p. 145; "from an old manuscript copy." b. Campbell's Albyn's Anthology, II, 28; "taken down from the recitation of Mr Thomas Shortreed," of Jedburgh, "who learnt it from his father."
THE LOCHMABEN HARPER—C
The Edinburgh Topographical, Traditional, and Antiquarian Magazine, 1849, p. 58; communicated by W. G. "from the recitation of a friend, who learned it many years ago from her grandfather," a farmer in Wigtonshire, who died in 1813, at the age of ninety-four.
THE BONNIE HOUSE O AIRLIE—A
a. Sharpe's Ballad Book, p. 59, No 20, 1823. b. Finlay's Ballads, II, 25, 1808, from two recited copies and "one printed about twenty years ago on a single sheet." c. Skene MS., pp. 28, 54, from recitation in the north of Scotland, 1802-3. d. Campbell MSS, II, 113, probably from a stallcopy. e, f. Aberdeen stall copies, "printed for the booksellers." g. Hogg's Jacobite Relics, II, 152, No 76, "Cromek and a street ballad collated, 1821." h. Kinloch MSS, VI, 5, one stanza, taken down from an old woman's recitation by J. Robertson. ["The earliest copy of this ballad hitherto found is a broadside of about 1790...."]
Motherwell's MS., p. 299; from the recitation of Rebecca Dunse, a native of Galloway, 4 May, 1825. "A song of her mother's, an old woman."
THE LAIRD O DRUM—B
Skene MS., p. 78; taken down from recitation in the north of Scotland, 1802-3. [see Herd 1776, in Appendix]