A question about 16 bit ADC representation
1) The offset and gain errors are likely to be larger than the difference between 65535 and 65534. You might not even see either of those at that precise input voltage.

2) The offset error is likely to be larger than the difference between 32767 and 32768. You might not even see either of those at that precise input voltage.

3) The gain error is likely to be larger than the difference between 20V/65536 and 20V/65535. The resolution might not be exactly one of those figures.

If you have a theoretical ADC, then these questions are easy to answer by consulting the definitions in the model. If you have a real ADC, then you need to read the specifications carefully, see how the terms are defined for that part number, and see what spread of values you can expect for zero input, full scale input etc.

Once you've linked to a specific part with published specifications, then we can fill in proper figures instead of using vague phrases like 'likely to be larger'.