[Dezion]

For your first question:
No, you may not enter your opponent's Secret Passage from the neutral zone. However, you may enter your own Secret Passage from the neutral zone.
For your second question:
You may not go from your opponent's Secret Passage to the neutral zone. However, you may for your own secret passage.

To sum it up: you can go in your Secret Passage anytime, but may only enter or exit your opponent's secret passage through your opppnents courtyard.