Q: Please explain, for the benefit of all, your "key test" for determining which song a key(s) should be in to be easily singable by the singer.
A: if the singer is me, i try it myself, and see where i like it the best. if it's someone else, you can guess based on having heard them before, what range they might like, and make some demos in the keys around there to see which one they like. or if you are actually with them you can play it with them to test