“the Kitniyos prohibition is about 800 years old,”
Only part of it. The prohibition on corn (maize) cannot be more than 500 years old, and as noted in the quote below, and the prohibition on peanuts cannot be more than a few decades old. Rav Moshe z’tz’l, who died in 1984, said that peanuts were not kitniyot. Some are now trying to asur quinoa.
” You cannot convince me that some time between the late 60s/early 70s until today that peanuts magically became kitniyot.”
This is actually proof for the Open Orthodox position that minhagim can and do change.