Then the question would be, a typo for what? Given that scholars have been studying this word for 1600 and coming up with various theories, if a typo was a plausible theory, they would have already thought of that.

Scholars do some pretty intense analysis of this stuff. See for example https://en.wikipedia.org/wiki/Papyrus_75 about how differences between different scrolls are tracked.

I would think so (that is to say a misspelling, not a typo as it wasn't typed), but on the other hand misspelling generally look like another word that fits in the context, but nobody seems to be suggesting a better word that looks like epiousios to put in its place.