I found the most satisfactory solution possible by experimentation:
At first, I was unable to modify or edit the categories and the items within them, because the new system works differently from the old system. The total number of games in all owned accounts are somewhat accurately tracked (accurate for currently logged-in account, not so much for the others).
Uninstalling Steam and cleaning the registry doesn't work; undesired settings are restored from the cloud upon reinstallation. The best I could do was delete all but the uncategorized groupings and/or the items within them [via context menu] (deleting all the items in a category removes that category).
I've been a Steam member since 2007 and a computer user since 1984. I had to make handwritten lists of owned games in the accounts' profiles and compare them to all the uncategorized games (a must!) before I could determine whether or not any were indeed missing. This was a process requiring many hours of boringly repetitive work, which I would not have had to perform otherwise (So thank you, Steam, for making this SO accursedly confusing).