Determine source of ⌘⌥\ being an alias of ⌘C
I found the issue. An Alfred hotkey (Preferences => Features => File Search => Navigation => Previous Path [no wonder it was so difficult to find!]) was conflicting with the 1Password hotkey. As far as I can tell, there is no mechanism to uncover such conflicts other than by exhaustive search.

As far as I'm aware, the hotkey I use with 1Password is the default. I'm amazed this conflict doesn't seem to be more widely-known.