Circuit to measure Voltage and Current (will it work?)
Your current source, I1, is probably upside down. 

At 20 mA there will be 10 V across R3. If Q1 is off then the voltage at the top of R3 will rise to its maximum possible. 

The 100k and the diodes will still protect the output.

I recommend that you turn off the grid when taking screenshots. This will improve legibility.