Don't know the answers to the milk questions.
Stevia isn't sugar and contains no sugar. It's the natural-world's aspartame--tastes sweet but isn't sugar.
Oil: coat the pan. You generally want oil to be somewhat hot before adding the things to it to cook. Makes the food less likely to stick and makes a tasty crust on the outside of the food.
Protein and fat are entirely different. Hunk of beef: mostly protein. Butter/oil: almost entirely fat. There are some who believe that high protein is hard on the kidneys, and others say that protein above what your body needs causes insulin levels to rise. Fat is insulin-neutral, so it avoids that effect, hence PB's recommendation that you eat as much protein as your body needs and relatively little carbs, and fill the rest of your caloric needs with fat.
Zone diet on and off for several years....worked, but too much focus on exact meal composition
Primal since July 2010...skinniest I've ever been and the least stressed about food