Malcolm,I don't think anyone is confusing "Riddles Wisely Expounded" with "The Elphin Knight" Child himself says "That the devil should supplant the knight, unco or familiar, is natural enough. He may come in as the substitute of the elfin knight because the devil is the regular successor to any heathen sprite"

Despite a degree of "romanticisation" about "Flower Lore" herbs have been known to be used in funerary and other similarly unromantic rites throughout Europe.
Like you said there is probably no definitive answer but that does not make the subject invalid for further discussion. I also hope that by providing the source of some of Lucy Broadwoods interesting conclusions (i.e. The Rev H Friend's "Flowers and Flower Lore" ) I am not guilty of simply repeating "received wisdom"