Baking Questions

Early during my gluten-free baking journey, I identified a number of questions that I wanted answers to. I have made progress in answering some of these; others, I still have some work to do. Here are some remaining questions I am working on.

1. In a Amy Perry blueberry muffin recipe, from a Carol Fenster book I found, she used a 3:1 sorghum flour/corn starch ratio instead of the more common 2:1. I cannot argue with the results; they were fabulous, but it made me wonder why. I assume that she first tried the 2:1 and was not happy with the result. What is the property she was trying to avoid? To state it another way, in what way were the muffins improved by reducing the amount of starch?

2. Likewise, sometimes recipes use soy flour, and sometimes they don't. I understand this is on a case-by-case basis, but when you try a recipe without soy-flour, what do you observe that makes you think "this could be improved by adding some soy"?

3. Other than baking multiple cakes, is there a way to decide how much soy to add? 1/4 c soy to 1 3/4 c sorghum mix seems standard in the ratios I've seen, but why?
4. Other than being commonly available and cheap, are there any baking advantages of corn starch over potato or tapioca? The amount of carbohydrate is not equivalent among them, but most gluten-free bakers use a mixture of all three, I've assumed for the taste, whereas Amy Perry Recipes use only one.

5. In the past, I have used soy flour as a replacement for egg in dry baking-mix recipes. Now that I am using soy flour for soy flour in recipes that use also use egg for egg, what do you think about the prospects of using soy flour for both? This one is purely speculative, I expect.