Analysing AC-Coupling differential opamp configuration
The common mode input voltage range for the OPA1642 is: -


  (V–)–0.1 (V+)–3.5 V


This means that if you have a power supply of 0 volts and 5 volts, the input range is -0.1 volts to +1.5 volts i.e. it will heavily distort the input signal because it is biased at 2.5 volts DC.


  OPAMP is OPA1642 rail-to-rail type


No it isn't. The output is specified as being "rail-to-rail" but the input range certainly isn't.


  Can someone help me analyzing this circuitry and how to modify it to
  reduce the gain? How to calculate high pass characteristics?


My recommendation is that you acquire a free simulation tool (like micro-cap) and simulate the circuit in order to further your understanding of how it works and what the frequency response is.


  Why is it necessary to have additional frequency filters when we
  already have a 22uF AC-Coupling capacitor in the end?


There may be some low pass filtering required and certainly, the 470 pF and 1.5 nF, offer some of that.