After going through your post, one gets a feel that your company might not be doing well financially. Since the old manager did not bring sufficient business, he was removed and new one was brought under a condition with certain business target. To fulfil her target she has been holding her team to ransom.
Coming to the solution to your problem. Let us discuss one by one.
You have been appointed to handle chat process for the international clients. However, owing to certain reasons, if your employers tell you to handle chat process of the domestic clients, then this may not be construed as major shift for the type of work that you do. In the interest of your company, you may show accommodativeness and accept it.
As far as working hours are concerned, are you working beyond 48 hours a week? If yes, then you are eligible for Overtime (OT) payment. Notwithstanding payment of OT, working beyond 54 hours per week is also illegal.
As far as change of shift timings is concerned, nothing much can be done. Employers are permitted to fix the shift timings as long as these do not exceed eight hours. However, in your case, the timing has been included in the agreement. Therefore, you may raise objection on this count. What type of bond paper have you signed? What is the legal validity? If the agreement is legally enforceable then provisions of Indian Contract Act, 1872 become applicable and either party cannot change the terms of contract unilaterally.
You may put up the application addressing MD of the company and explain your problem on your inability to do the night shift from 0000 to 0800 hours for the medical reasons. Attach the certificate from the certified medical practitioner about your disease. If your employer remains unrelenting then you may approach the Labour Officer (LO) of your area.
Please note that approaching labour officer is different from approaching labour lawyer. Latter is the redressal mechanism created by the government to solve the problems of the workers. It is free of cost. Whereas latter provides professional services on payment. Most of the employers do not show nerve to go against what the labour officer says.
There is flip side of approaching labour officer also. It could incense your employer and your employer might fix you in some or other issue and could terminate you. Therefore, if you make a complaint to the labour office, the logical step would be to ask for the discharge from the services. As such you have completed 18 months and quitting job should not be a problem.