Hello,
1) As regards your breaking the service agreement, you can be held liable only to the tune of the remaining time that you did not complete as per the Agreement. You have the option to offer the prorated amount in lieu of the relieving letter.
2) As you observed in most cases the company dues not pursue an employee who did not complete the term of Agreement.
3) You must take the offensive by sending the company a notice demanding your relieving letter. Send a detailed email before you go for a legal notice.