Is the Church's Nuptial Blessing, wherein she prays for and reminds the wife to be pure, faithful, wise in her friends, supportive of her husband and honoring of her vows a mental reservation? I mean, the couple just got married 45 minutes before, and the Church is already praying for the wife to remember her vows?
A prenup deals with civil divorce, not the sacrament. It's an agreement that if a civil marriage is destroyed by either party, then here are the financial penalties. Just like the Church has/teaches spiritual penalties during marriage classes, so the prenup is an agreement as to financial penalties. If the marriage stays together, then the prenup is meaningless, so it doesn't affect anything.