I've started re-reading Taylor.

The general thrust of his argument is to look at which features are present or absent in the various versions and surmise whether each feature is more likely to have been added at some point, implying that the versions that have that feature are newer, or to have been lost, implying that the versions that have it are older. One that he argues is old is the bottomless boat, which he says was a recognised punishment for serious crimes at one time. When the existence of that punishment had been forgotten, it is a plausible shift first to sailing away in a normal boat and then to exile by unspecified transport.

He surmises that the "where have you been?" opening in most of the Scandinavian versions probably crept in from a different ballad.

The 1650-ish version that was not available to him seems very different from all the others, British or Scandinavian. Its much greater length is unsurprising, many other ballads having been condensed in later years, but it includes little about the brother and (unless I've missed it) nothing at all about his having been murdered, about blood, about the impossible events attending Sven's return, or about what is being left to the wife and children. All it has in common with the later versions are the name Sven i Rosengård, the structure, and the intention to run away. Might it represent one source, with the fratricide and the impossible events having been grafted in from separate sources?