Great stuff, Jim.

If Giles Collins/Lady Annis is a burlesque of Clerk Colvill or even just derived from it, and if Clerk Colvill is a translation from the Danish we are realistically looking at a translation prior to say 1750. I'll have a closer look at English translations of Sir Olave/Elveskud. There are several. I have a Danish copy in Grundtvig which hopefully will be one of the English translations. I'll use 42A in Rieuwerts to compare.

Most ballad writers/translaters generally didn't put in unnecessary material of their own (Peter Buchan excepted), they usually kept quite rigidly to the stories/legends/facts as they received them, e.g., Bruton Town fuller versions adhere very closely to the 17thc English Translation from the Decameron.