Exactly, without the taboo on cannibalism the ritual becomes meaningless. You can't have the taboo without the thing existing at some point.
Oh and yes it literally does, that is the miracle!
You're talking about catholicism. In protestantism, it's "in memoriam" we do it.
And cannibalism was never an issue in any of the old religions. It's a basic human taboo, that only got broken when you go to places where food sources could be real scarce, like the Maori in the Pacific islands.
Human sacrifice is well known in Mesoamerican religions and a few instances of it happened in Scandinavia (we cut their throats and threw them in marshes, nobody knows why, so it's not part of the Asetro religion f.ex. - it's much older).
The Romans claimed the Celts and Carthaginians sacrificed humans, but we've yet to find evidence that I know of - and since both were enemies of Rome, of course they'll claim the worst of them.
And the only place I know it is mentioned in the Bible (or the Quran for that matter), is the story of Abraham and Isaac, maybe that tells us something about life in ancient Ur, but then again maybe not. And maybe Soddom and Gomorrah, but I haven't read that story...
Again, human sacrifice is a basic human taboo. Not something that forms the basis of any religion. In fact it was an old Roman insult to call christians cannibals.