My Vreg sense is IGN, just as in the factory design. I imagine that if you hooked that directly to the battery you would run down the battery when the car is off.
The diodes are a parallel path around the ammeter. They are not an additional series element in the path from IGN to battery. The diodes decrease the resistance in the path to the battery.
You are confusing voltage drop with resistance. Resistance is the voltage drop divided by the current. They don't usually plot that for diodes because it isn't even close to a constant ratio with current. But if you do plot it, you will find that the resistance drops as current increases, unlike a wire shunt. That is good for this application.
