What is ADC Supply Current in Microcontroller and how much current will a GPIO take
1) It means the current consumption of the ADC circuit within the IC, not from the input pin. Each ADC that you enable consumes 1 mA from the IC power supply.

2) That is the leakage current. 0.5 µA max., in or out. But if you use the ADC to sample a voltage, it gulps current in to charge its sample-hold capacitor.

3) It is implemented to enable the user to select both Vref-  and Vref+ for the ADC. Users might want full-scale readings between 0.5 V and 2.8 V instead of between 0 V and 3.3 V.