If One File=>Filename / If More Files=>Name of Parent Folder

Opus doesn't have any conditional logic, except within Rename scripts (which only see one filename at a time so aren't suitable either).

So to do this I think you'll need to make a button which passes all the selected items to an external VBScript (or similar) and have it decide what to do (which might include calling back into Opus to run some more commands via dopusrt.exe).

This simple script might be a good starting point if you want to try the script method:

[One Hotkey for 2 commands)