Niacin will probably be okay (eventually), but if you feel like you are having a heart attack/dying/or having an allergic reaction… you either might about to die or you might be having a reaction to the niacin. It can be decidedly unpleasant. You can get used to it, and with time it can take more and more niacin to trigger the reaction.
Clearly you are light on a number of nutrients and it looks horrible in terms of taste, mixablity, and stability at room temperatures. I’m not so worried about the things you are light in as you might be able to make them up from missing values in your other nutrients… but it does look a little daunting.
You are light on Omega 3s and 6s and the butter probably doesn’t help that at all. It looks like you are trying to stay cheap, but if you aren’t I like hulled hemp seeds (also called hemp hearts) for that. Others like Flax, soybean, or canola oil. I’ve said it before, but I’ll say it again … the Masa values are wrong for the B vitamins at least. Your oats and pea protien almost certainly have potassium in them. You are getting almost all of your counter potassium directly from a potassium salt… which can be risky in that it can throw your sodium potassium balance off with potentially severe consequences. So, I’d at least recommend that you slowly ease yourself into their recipe. At least your sodium is a bit high, that should help partially offset things.
The B12 being high is probably okay. I don’t think it is great to get nutrients (especially Calcium) from supplements… but to each their own.
Artificial sweeteners can be a problem because your body can trigger an “I ate sugar” response when there was none. I don’t know… mix a small batch and see how bad it looks/tastes?