“Just because a grain has been (relatively) recently discovered does not mean that it can’t fall into a category, and hence a prohibition”
Except that with kitniyot there is no prohibition. It is a longstanding minhag.
(FWIW I absolutely oppose dropping that minhag unless one actually moves to a Sefardic community and adopts all Sefardic minhagim and halachic rulings.)
“Kitniyos are harvested and processed in the same way that chametz is. “
Not true for rice — almost all of it is grown in flooded fields. None of the harvesting equipment used for land based plants would work. Think Pharoah’s chariots in the Yam Suf.