Sorry I missed your latest message. thus the late reply here.
Actually, what matters in the "Pipe Network Catalog Settings" is the folder location for the catalogs (i.e. a folder that contains at least one, or multiple catalogs.
When the dialog is to open, C3D would first look at at available partslist in the drawing and determine the partfamilies in the partslist are from which catalog in that folder. Then set that catalog as the selected one in the dropdown list. While it is possible multiple partslists in a drawing could be built on different catalog technically with C3D, I'd consider it is extremely bad practice.
For example, with the 2 drawing you attached here, when the "Catalog" dialog opens in my computer, the dropdown lists are all blank (no catalog is selected), because the catalog folder in my computer has a different folder location than the default C3D settings, and in my catalogs folder, there is no catalog name like "US Imperial..." or "Metric ...", all Catalogs in the folder of my computer are named differently. However, C3D only search the folder saved in the HKCU registry for partfamilies-matched catalog based on partslist in the drawing, when opening that dialog.
You can also verify it by renaming the "US Imperial...." catalog file to something else. Then if you open the dialog, you would see the default catalog changes to the newly named catalog, because the partslist's partfamilies match the ones in that catalog.
It would all become messy if the drawing has multiple partslist based different catalogs. Again, it is bad practice and should be avoid with all efforts.