Susan,
Had a close look at Douglas Tragedy. Not that many versions include the burial stanzas and the American ones seem to have mainly the rose from HIS grave. All of these finish with the true love knot forming. The earliest English version has the end cut off but we are told there was probably room for about 9 more stanzas. Pure conjecture but if you take the last verse of Percy Folio version and match it to the equivalent stanza in I, the stall copy c1795, it would suggest these burial stanzas weren't present in the earliest version.

Look carefully at the headnote to Scott's version in Child. It says 'the three last stanzas from a penny pamphlet and from tradition.' By 'penny pamphlet' one presumes he means the stall copy, I, which he didn't have access to when he made the notes. The stall copy indeed finishes with the 2 stanzas 18 and 19 in Scott and Scott's version follows the stall copy pretty closely. Scott's final verse about 'Black Douglas' is found nowhere else in this ballad's tradition. In fact no other version goes beyond the lover's knot. I know some uses of the motif in other ballads have someone sever the knot, but not in this ballad. I leave you to draw your own conclusions from this.