1. Deffo not Nadal. Isner always a good shout when it comes to serving.
Nadal and Djokovic second serves are made look better because of their superior ground game. The serves itself are very ordinary
The previous poster thought Federer might have the best second serve among current players. Both you and the previous poster also find weaknesses in the Djokovic and Nadal second serves. Fair enough. On the third question, about using just a first serve even on the second serve, perhaps those with high double fault totals, like Goffin or Verdasco, maybe these players are doing that. Or maybe they just have erratic second serves.