Audio power amplifier features identification
1 - 18kΩ resistor together with 2.2µF capacitor for a high pass filter. 
The Fc frequency is equal \$F_C\approx\frac{0.16}{RC} = 4\textrm{Hz}\$ 

Additional 1kΩ resistor form a low pass filter together with 1.2nF 

\$F_C\approx\frac{0.16}{RC} = 134\textrm{kHz}\$. And this filter stops any RF signal, so the RF signal is not being amplified by the amp.  

2 - In the VSA stage the current is transformed back to voltage. 
Because due to large Miller (100 pF) capacitance. The VAS stage work as a transimpedance amplifier (current to voltage converter).  

3 - Because we want a dominant pole compensation (big Miller capacitance).
And in cascode stage Q7 do not provide any voltage gain (no voltage gain no Miller effect).  This is why we put 100 pF capacitor at Q8 collector.  

4 - This resistors separate the output stage (emitter follower) from the VAS collector. Because sometimes the output stage can cause HF oscillation. And we can stop this oscillation from happening by adding a base stopper resistor. 

So the VAS will always see a "real" resistance (emitter follower can have a negative input resistance).      

http://www.ti.com/lit/an/snoa737/snoa737.pdf.

5 - The voltage headroom is the answer.
The voltage swing at the VAS stage (Q7, Q9) is much lower then this +/-55V. 
So to be able fully "diver" the output stage. The input stage + the VAS stage is supply from the higher supply voltage.