Hello,
There is a version of the story of Doon, Olive and Landri in the English version of the Karlamagnus Saga: The saga of Charlemagne and his heroes (Toronto, 1980), chapter II. However, since it is no longer in print, you would probably have to look for it in a university library.
I like your "descendants of Drogo" idea very much!
Have fun,
Ruben