@asha,
1-Can they get false certificate from some doctor and obtain divorce on this ground?
They cannot get false certificate from some doctor. No doctor will give certificate if a person is not his patient. If anyone adventurous to such an extent, he will be doing it at his own risk. But the husband may move an application in the court seeking your medical examination proving the alleged mental problem. As you are healthy, this medical examination will also go in your favour. Further, before giving such direction to send the respondent before the medical board to find out her mental health, the court dwelve upon such question at length and there is more possibility that his application will be dismissed.
2-what should i do to proove that i was forced out of the house and that i am not staying in parents house with my own will?
You simply write down the abuses perpetrated against your body and soul by your husband and his relatives (if that is so) and send the letter saying that due to those reasons you were forced out of the matrimonial house. This letter you send it by registered post. As per the DV Act, the definition of harassment has been expanded and by that not only beating and torturing but also emotional and economic abuse also are part of harassments and the women have got a right to approach the metropolitan magistrate to seek the remedial measures. By sheer power of physical strenth, shamelessly the husband and his relatives push the helpless lady out of the matrimonial home, the court will, as interim relief, provide her "residential rights" in the matrimonial home.
You are correct that it is not easy task for a tortured wife or battered wife in matrimonial home to prove the fact in court of law. But no court demands the traces on the body of the wife to prove the fact of harassment. Just slaping a wife does not leave a trace on the face nor requires medical attention. kicking on the some parts of the body also does not leave the trace on body. But it gives unerasable scars on the psyche of the person. These things sensitive courts understand and come to the rescue of the ill-treated woman. Harassment in Section 498-A or DV Act does not demand the proof of body scars. The pre and subsequent behaviour of the wife and husband after the wife thrown out of matrimonial home will give clear indication to the court to find out the truth. Hence, do not worry about quantity and quality of evidence at this stage and if a wrong is done to you, and if you wish to fight it out, consult a good and woman sympathetic advocate near your area and he will guide you in a proper way.