I have tried, with difficulty, to find good Halachic information about kitniyot on Pesach for those who eat kitniyot (there's plenty of information for those who don't), specifically about whether it is possible to buy kitniyot on Pesach itself. In most years, I do buy before Pesach starts, but I have my mother visiting who has a very specific diet that requires much of her protein to come from beans and other legumes, and I realize now I do not have sufficient quantities in store.
I know there are opinions that require checking of rice 3x, for example, before Pesach, and others which do not require checking at all (but require removing foreign grains if these are found mixed in with the rice). They base their decision on the superior processing of grains and legumes that exist nowadays, and the efforts of processors to segregate foods due to allergens.
On a related point, is part of the permissibility of kitniyot based on any traces of hametz being nullified as part of Biur Hametz on Erev Pesach?
Since there is a dietary need in place here, am I butting up simply against a minhagic prejudice (which doesn't actually apply to me, since I'm Sephardic) or is there a significant halachic concern in play? Is there wiggle room to allow me to provide for my mother's needs?