Purpose of capacitor and diode in analogue input protection circuit of ADC?
A partial answer

1 - Usually a low value capacitance is placed as close as possible to the connector for better EMI and RFI.

2 - Me neither

3 - Also, don't know. Although I guess it is a very slow filter, since the signals are industrial and relatively slow. What confuses me is that it was placed upstream of the resistor divider.

4 - The offset is done because the input ranges are symmetrical and the ADC input is single ended.