Question 1: How do you make very accurate resistances with very large and very small values? This is straightforward. If you want to make a 1 MegOhm resistor, a 1% tolerance implies that the actual value could plus or minus 10,000 Ohms. If you want to make 1 milliohm resistor, then a 1% tolerance implies that the actual value can be plus or mins 10 micro ohms. The point is that there are limits on how accurately a given value of resistance can be fabricated when you get to both very large, and very small values.
Question 2: When you try to make a multirange instrument from the 0-1mA moving coil meter, the further away from that range you go the bigger the resistors will be for the voltage scale, and the samller they will be for the current scale. When the ratio of the series or parallel resistance with respect to the 500 ohms of the coil becomes large enough, or small enough it has a correspondingly greater impact on the resulting measurement.
I don't know how much plainer I could be. I think its time for me to take a walk as well. My Basset Hound Blue needs to take a whizz.