Effect of increasing nitrogen concentration in the synthesis of ammonia
The final concentration of H$_2$ is lower than the concentration of NH$_3$ because for each NH$_3$ you produce, you need 1.5 H$_2$.  So the rate of consumption of hydrogen is higher than the rate of production of ammonia.

Here you can read about the multiple steps in the synthesis of ammonia, also called the Haber-Bosch Process.  As you mention, by increasing the concentration of nitrogen, you can increase the rate.  The rate limiting step is separation of the nitrogen atoms in N$_2$