re the usage of "the red rose and the briar", "leaned her back against an oak" etc., I referred in aprevious Guest post to the collector William Motherwell. It is often the case that Introductions are better than the books. But not always: in his 1827 Introduction to "Minstrelsy", Motherwell points out that -
"This uniformity of phraeseology...which pervades our ancient ballads might appear to argue a poverty of expression...the use of such common places is abundantly obvious. They not only assisted the memory....but served as a kind of groundwork. With such common places fixed in his mind, the minstrel could....rapidly model any event which came under his cognizance into song. they were like inns or baiting places on a journey.....they were the general outlines of every classof human incident and suffering...They were like a commodious garment that could be wrapped expeditiously round every subject of whatever nature or dimensions."
So - from a contemporaneous source, phrases did not link songs or even suggest any association.
Hope this helps