Q358 (I think) - Re: A swashbuckler's Opportune Parry and Riposte - what happens with Natural 20s?

Quote Originally Posted by The SRD states
The swashbuckler makes an attack roll as if she were making an attack of opportunity; for each size category the attacking creature is larger than the swashbuckler, the swashbuckler takes a –2 penalty on this roll. If her result is greater than the attacking creature’s result, the creature’s attack automatically misses.
And attack rolls automatically hit on a natural 20. So what happens in the following three scenarios?

i) The attacker's roll is a natural 20 but the swashbuckler's parrying roll has a higher total though not a natural 20.
ii) The attacker's roll is not a 20 but the swashbuckler's parrying roll has a lower total but is a natural 20.
iii) Both rolls are natural 20s.