Just to be certain - do I have this right:
Using a rapier (1d6/18-20x2)
1. If I roll any of 18, 19, or 20 I have a threat of a crit, I must roll again and succeed in hitting against the opponent's AC to qualify for the x2 damage. The x2 damage is not a single damage roll x2, but two separate rolls calculated using all modifiers.
As an example: roll an 18, resulting in a crit threat. Roll again and get 15 which hits. Roll 1d6, add my strength modifier for damage, do the same again and add the two results to get total damage.
2. Also, a straight 20 does not mean an automatic hit with a crit range weapon, correct?
20 is always an auto-hit as far as I know.
With a weapon with larger threats, the 20 is still an auto hit, while the 18 and 19 are not auto-hits, but still possible crits if they hit normally.
So, in your example, your 18 would have to hit its AC, THEN you'd confirm the crit, which would also have to hit its AC, then you'd roll 1d6+strength twice.
And if I failed to confirm the crit after rolling I would still get the regular damage right?
Yep.