The blue LED is conected to the IP5306 charge controler and therefore is not influenced by the firmware.
easy solution would be to put electrical tape over it.
I didn’t find any Dokumentation or setting concerning gain for good reason. Gain is set by the Amp and not the I²S audio source.
For the Protoboard you can change it by changing resistor. A good explanation is given by Diyi0t
Alternativly I tried to mess with the Equalizer. you can try it out your sellf:
That should give you an ideea how it influences the audio output.