“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.