hmm, as I like to invent my own recipes I can see this one needs a little help sorting out the instructions. I would use 1.5 tsp baking powder and .5 tsp soda. the soda is used to balance out the acid from the lemon juice. The powder has baking soda in it but a little extra is needed when using acidic ingredients.
For a lemon cake I would use millet flour. Buckwheat would be dense and the nutty flavor may not taste so good. Coconut will not work here as it will be way to dry! Plus millet will give it a pretty yellow color! I dont know what Stevia she is talking about as I would never use 1 cup. Maybe she is using a blend. If you are using stevia extract you prob wont need more than 2 tsp. Any more than that and it will have a bad stevia aftertaste. hope this helps with the instructions.