It would, of course be wrong to suggest that such an individual was unique, which I didn't. But they did exist. If humans and chimps (for example) have a species that is our common ancestor, then necessarily there was at least one individual in that species which was our common ancestor, because two individuals can't be related unless they have at least one common ancestor.
I fully agree that there would be such an individual, and probably more than one.
But is that really what we are talking about when we reference a common ancestor? I would say no. That is not what we mean. When we say that humans and chimps share a common ancestor what are we really trying to describe? Well, we are ultimately talking about a speciation event where a single population split into two populations. The common ancestor would be the ancestral population that spawned the two separate lineages.
When we look at the fossil record we are forced to use the same concept. We have no way of determining direct ancestor/descendant relationships between fossils, barring the discovery of DNA in those fossils. We are forced to describe relationships through common ancestry, and once again these relationships are referencing an ancestral population.
The OP asked, "Despite all this, when was the last time anyone described to the public or expected the public to assume that the term "the Common Ancestor" was actually plural, not necessarily distinct, and possibly metaphorical in nature?". That is a great question. Perhaps we SHOULD stress what we really mean by common ancestry. It is trivially true that we can trace the lineages of any two organisms back to an individual who is a common ancestor, but that is not really what we are describing. I think it also causes confusion when we talk about mitochondrial DNA and y-chromosome MRCA's. It can cause people to think that ALL of our DNA came from those two individuals.
So should we be using something like "common ancestral population" instead of "common ancestor"? Perhaps we should consider it.