The sheer volume of material Buchan produced would have daunted even him. How sure are you he wasn't part of the plot? You need to look at at least a number of ballads in detail and do some serious number-crunching to be aware of the extent of Buchan's creative capabilities, but the easy one is that most of Buchan's ballads are at least half as long again as anyone else's and they are 'eked out' with Child's 'nauseous repetition' overdoing the incremental repetition that is naturally found in the ballads, and overdoing the commonplaces, and adding in completely fresh material found in no other versions. Scott did this of course, but nowhere near to the extent Buchan did it.