In the book Les miserables how was it easy for the criminals to escape from the jail when they did?

Also I have more questions

9. How was it easy for the criminals to escape from the jail when they did?

14. How long has Marius been gone from his grandfathers' house?

15. What does M. Gillernormand suggest Marius do rather than marry Cosette? And how does M. Gillenormand Know of her?

20. What does Gavroche notice about the pistol he found in front of the secondhand shop?

21. Who identifies Javert as a spy at the barricade?