Long term stability of okara flour
The safety of the flour will depend on water activity. We cannot know the water activity of the flour produced in your kitchen, but 1) home dehydration is known to generally create shelf-stable products, and 2) if you could create a powder, it is pretty certain that you don't have enough moisture for bacteria to survive. And while molds can survive much lower amounts of water than bacteria, the good news about them is that you will notice dangerous levels of mold itself, while bacteria can be dangerous before you notice rotting. So I would tend to treat it as any dehydrated plant matter and consider it safe.
The oxidation will certainly happen. It does not fall under food safety, but is considered a quality issue. Oxidation is common in flours. Commercial wheat flour is defatted and has antioxidants added, but other types of flour, especially nut flours and nut meals, are eaten oxidized, unless you grind them fresh for every dish. For any quality issues, it is up to you to decide whether they bother you.