Regarding point one, the 5" barrel is easier to control and more difficult to conceal than the 4". That means that everyone would need 5" barrels to compete. Of course the 4" is easier to control and more difficult to conceal than a 2" so everyone needs a 4" barrel to compete. No logic, just a decision to draw a line somewhere. Prior to 2002, 5" barrels were allowed in IDPA.
Regarding point two, the steel guidrod is not a stock part and is heavier than the stock part so, in theory, it could offer an advantage over the plastic guide rod. Therefore, only allowed in ESP.
Point three - Please see my previous post...
