Impedance frequency response
So my question is why did we equate Z=R=100 to  \$ \big( \frac{1}{j\omega C} \big)\$ only? Why not to \$Z_{inductor} + \frac{Z_{resistor} \cdot Z_{capacitor}}{Z_{resistor} +Z_{capacitor}}\$? 


You're right. You should use the latter to calculate it correctly, but the first approximates it very well and computes more easily.
The effect of the inductor is present over the whole frequency range. But it starts to be significant at higher frequencies, so, at lower frequencies its impedance can be approximated by zero.


  Why did the impedance has decreased after this mentioned point?


The impedance of the capacitor decreases with increasing frequency \$ \big( \frac{1}{j\omega C} \big)\$. It is in parallel with the resistor. So the effective impedance of the will decrease with increasing frequency.
The effect of the capacitor is also present over the whole frequency range. But it starts to be significant around this mentioned point.