Whether or not buckwheat is a grain or not shouldn't matter. It isn't about whether or not something is a grain - just because it's a grain doesn't make it unhealthy. What should matter is the nutrient density of the food and the toxin load of the food.
For example, white rice and masa harina (nixtamalized non-GMO corn flour) are both clean. There is virtually no toxins. Compare that to something like nuts, which are Primal but contain high levels of phytate, moderate levels of lectins, inflammatory fats, poor quality proteins and are often allergens. Dairy isn't considered paleo, but it is one of the most nutritious foods on Earth and has a lipid profile superior to any meat I'm aware of and the best quality protein in nature.
Buckwheat isn't technically a grain, but it is high in phytate and prone to going rancid due to the polyunsaturated fat content, which is relatively high. The fat content is higher than all purpose wheat flour, and most wheat flour is rancid on the shelves to a degree. Logic dictates buckwheat flour will probably be worse.
I see you can't have tapioca starch. That is unfortunate because it is very clean. However, white rice flour, non-GMO masa harina, potato starch and arrowroot starch are all very clean - much more than buckwheat. Coconut flour is acceptable as well, but it's a bit tougher to digest, expensive and much trickier to use in recipes.
You can consume that if you want and it's not toxic, but it's mostly all empty calories with super high palatability. You may be avoiding gluten and rancid fats, but you're still likely to overconsume empty calories, similar to eating a spoon to a bag of sugar or jar of oil. It is best to just try and avoid flours altogether, "Primal" or not. The occasional treat is one thing, but when it's several times a week...it is a slippery slope very quickly.
Don't put your trust in anyone on this forum, including me. You are the key to your own success.