People seem to be confusing Riddles Wisely Expounded (Child 1) with The Elphin Knight (Child 2). That often happens. Essentially, the Devil belongs to the former, the elphin knight to the latter.

In the former, a knight asks a lady a series of riddles, which she solves; sometimes he is represented as the devil, sometimes not. A 15th century form, Inter diabolus et virgo, found by Child too late for inclusion in the main entry but added in an appendix, suggests that the "hero" was a devil from the start.

In the latter, which Child took to be the ancestor of both the Scarborough Fair and Acre of Land song groups, knight and lady set each other a series of impossible tasks. The earliest texts cited (broadsides, as mentioned above) describe the knight as "elphin", and this is retained in some later Scottish examples from oral currency. In most others no supernatural element is mentioned. The Devil appears in the titles given to two Scottish examples by their collectors/editors, but without any internal evidence, so these may be editorial (though Child seems to accept them at face value). Finally, Child prints a text found by Baring-Gould in Cornwall, where the man is the woman's dead lover. Although it may be authentic, there are signs of editorial intervention of the kind Baring-Gould so often engaged in, so I wouldn't base any theory on it.

Much as I admire Lucy Broadwood's work, she was far from immune from the romantic tendencies of the time. Folklore studies a century ago were full of magical herbs and Grail Knights, most of which were probably quite imaginary. This, after all, was the period when things like Wicca and Neo-Paganism were being invented. That's not to say that there may not at some point have been some significance to "herbal" refrains in some contexts; but we don't actually know that, and we don't have evidence that the people who sang the songs attached any particular meaning to the refrains on the whole (though they may have made a guess if asked).

The subject has come up before, but mostly without very much useful information being posted. Typically, people have just quoted (usually without attribution) half-digested tidbits they've heard somewhere or other, presenting them as "fact". Lay the bent to the bonny broom has been discussed at much greater length, but often at cross-purposes by people who thought it belonged to The Cruel Sister. That particular phrase is as likely to have a sexual as a magical meaning; but sex was a bit frowned on a century ago. The romantic approach was "nicer", and is still preferred today by people who rely for their information on outdated sources.

I doubt if anyone will ever come up with a definitive answer. People like Miss Broadwood and Lowry C Wimberley (Folklore in the English and Scottish Ballads, 1928; you'd enjoy that) made some very interesting guesses, but in the end that's all they were. They get repeated as received wisdom, but on the whole students of the subject are more cautious today. Without real internal evidence, all we can reasonably say is "perhaps; and perhaps not".