Module:category tree/topic cat/Etymology

Category:Biblical derivations. Per its text, it should be empty, and near as I can figure, the only to way to empty it is to remove it from Module:category tree/topic cat/Etymology and add the thing it redirects to, which is what I was trying to do.

Not at all. What you did was, in fact, to add categories named Category:en:Terms derived from the Bible by language and such, which are not what you want.

The only way to empty the category is by editing the pages it contains and remove the category from them manually.

Um, pretty sure that's not the case, because the category isn't on any of those pages; it is categorized that way due to the module. And, since redirects shouldn't really be in the category tree anyway, Category:Biblical derivations should be removed from the module even if nothing is there to replace it.

But Category:Terms derived from the Bible by language already exists and has existed for a while now. So I'm not sure what the "rename" you're performing is supposed to achieve. As I said, the only way to empty out the subcategories of Category:Biblical derivations is to edit each entry and rename the category in the entry.