Yes you can substitute it for rice and its what I’ve been recommending to everyone as an alternative to rice for about a year on the forum! It tastes great and has better nutritional properties than rice. I think you shouldn’t worry about carbs as long as its a prebiotic food source.
Buckwheat comes in a few different forms; the flour, raw grain (its not a grain, but looks like one), and ground up buckwheat (cream of buckhweat).
Raster’s buckwheat recipe: 1tsb of olive oil, 1/2 cup buckhweat, 1 cup water, rosemary, basil, sea salt.