I've started setting up subsidiary folders in library, but I made at least one error and want to correct it if possible.

in this outline, I want to move loony cast down under loony book and maybe retitle it? i'e also like to delete the  line for "mall". but deletion doesn't work and no minus sign has appeared yet for this, so?

drawn

The option to remove folders isn't present because there may be assets used by scene you created still in the folder. The morph data files (PMD) don't show in the library but they may be used by scenes.

Deleting a folder has to be done at the OS level. Right click the folder in question then Show in Explorer (Finder). Examine the folder in explorer. If it's truly empty it's safe to delete. Trash the folder in Explorer. When you go back to Poser refresh the library folder above where you deleted the file.