Paul, I'm sure the answer would be that you can suit yourself about the melding of crafty farmers, clever girls and wicked/outlandish knights. But I'm confused by the conflation of 'The Unquiet Grave ' and 'The Twa Brithers'.
Surely the first is about two sweethearts parted by death, while the second is about fratricide.
I'm having difficulty following your reasoning here.
But perhaps of more interest is why you might want to move beyond the recognised 'originals' to some composite/amalgamation? [I know there is much (ongoing) discussion about the 'folk process', but I know that I find personal satisfaction sooner in the 'polishing' of a version of one of these ballads, rather than a combination of several.]