Richie,
You're talking about an appendix of pieces that employ a small section of a Child ballad. If you feel it necessary to include these rewritten ballads like 'Masterpiece' that employ a motif from a Child ballad then you should include any further rewrites like 'White River Shore'. They are every bit as valid as 'Bold Dragoon'. They're only going into an appendix. No-one nowadays will see any of them as Child ballads. Just make it clear that these are appendix pieces (i.e., related, however tentatively) and therefore separate from full descendants.