Steve, Douglas Tragedy is an amalgamation. It seems to have drawn from multiple sources:

I understand there were two traditional singing versions of Earl Bran courtesy of the Laidlaws at Sir Walter Scott's disposal. That's a good start.

You mentioned Jamieson's translation of Ribold & Guldborg, however, I don't see any direct relationship there-just the detail of the 7 brothers. You have to remember that just because Jamieson published, doesn't mean he's the only person capable of translating Danish. Whether brothers or King's men pursued is probably not as big a deal as I first thought. It is more likely that the publication of the Douglas Tragedy inspired the translation. Who knows? In it's time, "Minstrelsy" made a big splash.

Fair Margaret & Sweet William were definitely invoked as the names for the unhappy couple. The name of Douglas was invoked as well. "Child of Elle" in Percy's Reliques was used (as you point out they have text in common). Sir Walter Scott was a great follower of Percy and had plenty of exposure to "Reliques."

At the time the Douglas Tragedy materialized, the motif was in circulation, so the person who performed this synthesis attached the motif as well. Because it says "kirk" and "quier" and not "higher chancel" and "lower" (as consistently appears in Fair Margaret and Sweet William), I'd say he took it from Lord Lovel.

The motif itself in the Douglas Tragedy is likewise an amalgamation. Half traditional and half variation.

It's a contrived piece Steve. Prince Robert is contrived as well. I think Sir Walter is a more of a carnival barker than a song collector. We should talk about Fair Margaret and Sweet William.

Actually 1685 sounds about right for Fair Margaret and Sweet William. Here's a link to view the Douce ballad:

http://ballads.bodleian.ox.ac.uk/view/sheet/30012

Note where it says "MS annotations of title: To the tune of Lord Thomas or Lord Thomas he was..."