SOLUTION FOUND!
Frustated, i rechecked my CCleaner Applications check list. I was using Opera, so left Firefox cookies unchecked. In a why-not mood, i checked Firefox. And then used Firefox to see if it was retaining the same sites cookies. Success!
My extraneous Firefox cookies are now vaporized.
My Opera cookies remain indestructible but I'm jumping ship to Firefox anyway, so i'm content with not solving that mystery. [Opera users: your cntrl+shift+L bulk downloads, Opera's clumsy equivalent of Firefox's Downthemall, may lose files. Mine does, an obviously fatal bug.]
Anyway- it seems it was a bug in Ccleaner.
Hope this helps others...