Baking – Using beef fat for baking

Flour is basically a mix of gluten and starch (about 10% gluten to 90% starch for all purpose flour, the ratio varies for other types). Whenever a baked good asks for the addition of pure starch, it is made under the assumption that you have no easy access to low-gluten flour types. Its purpose is to reduce the gluten-to-starch ratio.

Gluten makes dough tough and is also the substance which makes it hold together. With less gluten in your dough, your cookies will be less tough, have more of a tender texture. They will also crumble more easily.

Instant pudding is an ingredient which is more common in some kitchens than pure starch, so it is used as a substitute. It usually has sweeteners and aromas in addition to cornstarch. It is less controllable than using pure starch and adding the best amount of sugar and the desired flavoring itself, so it trades quality for convenience. In my experience, recipes which call for instant pudding also cut corners in other respects, and are unlikely to be of high quality overall. So I prefer to not use them. If you want the low-gluten type cookies, choosing a recipe with cornstarch (or directly cake flour instead of a mix of AP flour and starch) will probably give better results.

Not really, unless you go for a different carbohydrate with physical properties similar to refined sucrose. And if you are removing the sugar for dietary reasons, you're probably not winning anything by doing the substitution.

Artificial sweeteners and stevia are just that - a sweetener, not a sugar. They can only be successfully used as a substitute where table sugar is used as a sweeteners. For example, they work well in a coffee or a homemade lemonade.

In baking, sugar is not used primarily to make the batter sweet, in fact it often makes it too sweet for my own taste. Its purpose is to provide structure. It has unique physical properties it shares with other short carbohydrates, but not with non-sugar sweeteners. You cannot substitute sugar for something which is completely unlike sugar and expect to get the same result.

If you are very adventurous, you can try using soluble fibre in place of sugar. But it's hard for a home cook to 1) get a supply of it, and 2) tweak a recipe such that it works well enough texturewise.

If my assumption is wrong and you are not counting calories, it becomes easier. Adding pure glucose or fructose will produce a recipe which still needs a tweaking, but can potentially be made similar to a sugar-containing one. Then you should also leave out the stevia, because it will make the bisquits overly sweet.