(Tbh this is mainly about nadal, because his style is a lot defensive than djokovic who hits his shot much flatter)
They play defensive tennis, go for less riskier shot, opting just to get the ball back instead of trying for a winner. They leave a greater margin of error in their shots and as a result, their unforced error counts are always lower. Therefore its not surprising that in a highly pressurized situation (i.e. match point down/deep into the 5th set), the nerves in their body would affect them less than someone who plays near the lines or hit flatter shots.
1) I don't see how you can logically come to the conclusion that defensive tennis players are a lot more likely to loosen up when in seemingly inescapable situations. If anything, they're more likely to do what they do best
2) Djokovic's fearlessness in certain situations speaks for itself. 2011 was a clear example of that.