1. re VSS vs VSSA, you found a typo. It should indeed be VSSA as this agrees with the TRM and other documentation.
2. The SAR ADC is not a resistive ladder, it a multiple-level switched capacitor type.
There is another problem in the datasheet, no mention in
absolute max ratings the relationship between Vdda and Vdd
nor Vssa and Vss.
That should be added.