derek500, in this 2015 post, gave me the solution. See the remaining posts by me in that thread for further discussion. Although I'm still using Retrospect Mac 14, I routinely use that undocumented dragging-in-the-Details-panel-of-the-script-Summary trick whenever I Remove and re-Add a source. See further posts by me in that thread for an explanation. There's a limitation for local Sources because you can't easily Remove them, as discussed here and in posts that follow it. In any case, the first paragraph of this following post is my explanation of precisely what is the nature of the "alphabetical order" that Retrospect follows in backing up Sources within a script.