“If is most certainly assur to own chometz on erev Pesach after chatzos mid�oraisa.”
That is indeed the halachah but as all of us who have just finished tractate Pesachim know, that was not a unanimous opinion.
“Why doesn�t kitniyos before chatzos have the status of 2 derabanons which is most probably permitted.”
Al pi halachah kitniyot is mutar both deoraita and derabbanan. It is a minhag. And what constitutes kitniyot isn’t even clear. The Rema has a list but we don’t seem to follow him.