just trying to understand digestion

carbohydrates?  assuming that all carb feedings will be taken with protein as well (and some fat), when is insulin released, once in the stomach or after being released from the stomach?  what is the bearing of this on glycemic index?

is there a process special to protein that, if taken in excess, converts to bodyfat?

also, fiber is indeed a carbohydrate, but does it all completely digest or is some/all passed through the body undisturbed?

Insulin can be released when you smell food.  But the lion's share if volume-dependent, so when the blood sugar starts climbing.  Volume of food always supercedes the GI value. 

Protein causes insulin release as well; again, volume-dependent.  But without the carbs, there is little to convert to fat and protein being both more thermogenic (taking more calories to digest and assimilate) and the fact you won't typically binge on it, is not the greatest threat.  BUT it is one overlooked by many.  To avoid carbs and fat many eat way too much protein and it does slow you down. 

Soluble fiber is used...just more slowly; a complex carb.  Insoluble fiber isn't used until the large intestine converts some of it into fat to be reabsorbed through the hepatic portal system and into circulation (if not used first).  The insoluble fiber is a very small amount of the food you eat, though - add it up gram for gram.  Small player and you need it for bowel function.  As you know, I factor it into total carbs for purely semantical tracking reasons.