Child 002! The Elfin Knight/Scarborough Fair.
Checking in on what Child wrote, he contextualizes this one among many stories and songs of courting couples giving each other impossible tasks to either seal or avoid a formal union. It's folktale type Arne-Thompson-Uther #875, The Clever Farmgirl.
Of all the versions, Scarborough Fair is probably the most famous, likely the most famous of any Child ballad for that matter. The most expansive version of the tale (and the most interesting to me), though, is the oldest known one, which Child puts as variant A : The Elfin Knight, sourcing a broadside printed "about 1670" now in the Pepys Library, which you can see here.
In it, our story starts with an elf knight standing on a hill, blowing a (magic?) horn all over the place, and a young maiden in bed nearby hears it and wishes the horn was in her trunk, and the elfin knight in her arms.
No sooner does she say this but the knight is at her bed, telling her she's too young to marry. She rejoins that her younger sister was married yesterday, and in turn the elfin knight demands a courtesy before they marry- a shirt without any seam or needlework (just as in Scarborough Fair). In return she demands a courtesy of her own- an acre of sea-level land eared with his horn, sowed with corn, and a cart of stone and lime trailed by a robin, and the corn then barned in a mouse-hole, thrashed in a shoe's sole, winnowed in the palm of the hand and the glove, then taken over the sea and back again.
So if my interpretation of 17th c. Scots English is correct (questionable!), the young woman either wins the wit-contest or it may be considered a draw, but the elf-knight proclaims he would still sooner die than give up his plaid (and in context this somehow means to commit to marrying the young woman) because it wraps his wife and seven children already. Hearing the knight is otherwise committed to a family already, the maiden decides stay a maiden, and to keep her own plaid.
So, this is a wild story that creates as many questions as it answers!
1) Do any other variants add additional story that this one might be missing?
Not... significantly. Child's variant D from Buchan's Ballads of the North of Scotland is largely similar to A though lacking the last 3 stanzas, adds verses that specify that the young woman is nine years old and her sister eleven (oof).
Most versions are pared down to just the impossible tasks, which vary in form, and there are also a lot of of interchange with the refrain- "parsley, sage, rosemary and thyme" is classic, sometimes with other herbs, sometimes warping into "every rose grows merry in time" and "where every rose grows merry and fine". Similarly, "Scarborough Fair" becomes "Rosemary Fair" or "Whittingham Fair".
The fun variants: there's one where the elf is instead an old man, and another that Child suggests is descended from that one, a ghost story where, having accidentally summoned her deceased love by wishing him with her, the protagonist must answer his quest well to avoid going with him into death.
I'm also very excited about the Norma Waterson/Eliza Carthy version where the elfin knight is a devil instead. It was collected by MacColl and Seeger from a Scots Traveller named John MacDonald in the 1950s so I don't think Child ever saw that one, but he interpreted the old man as a devil-figure and mentioned that a devil would be a much more natural archetype for the riddle-giver. I quite like the rollicking, brazen melody and the "she didn't like his manners since he didn't think to ask" line Waterson/Carthy added for their interpretation.
2) Why do so many people discussing the song think it's either a chill love song or view the knight as the aggressor, when it's clearly the maid?
As far as I can tell those last several stanzas did not survive in the oral tradition, and the beginning few are rare.
3) What's with all the plaid?
Here's a very horny drinking song which shares the same refrain.
4) Why is it like that?
To me, the oddest part is that it's not a love song, nor a tragedy, just a song of an awkward child-woman trying to sleep with a married man that rebuffs her advances, first with a fanciful rebuff, then a more prosaic refusal. Not unknown in the folk tradition (it puts me in mind of The Soldier and the Lady) but an odd focus for the framing of the riddle-tale, for sure: I'm not surprised that the last few stanzas were dropped.
Child's opinion was that the first 3-4 lines and the last two are confusing and must hail from some other, lost ballad. He specifically noted that the first 3-4 lines of the song form the beginning of Lady Isabel and the Elf Knight (Child #004). He also says that elves giving riddles is unknown in Northern tradition, and that only the English version of this story has a supernatural protagonist, though he goes on to note a number of riddlers from myth and legend.
Here are some more versions:
