1. Chatting with your girl friend can not be a ground for filing 498A complaint or DV case. Her thrats are unfounded. Moreover, you can file a police complaint against her for hacking your password since hacking other's password, even if he is the husband, is an offence as per IT Act,
2. Court will not ask Yahoo to submit the said record. Moreover, Indian court has no jurisdiction on Yahoo to pass such order/direction,
3. No. Only divorce case can be filed showing the chat record accusing extramarital relationship which can be contested fittingly by you. The moment she files the said chat record under affidavit, she will be liable to be prosecuted under I.T Act for hacking your password and illegally entering in to your email account,
4. No such case will stand on your GF on its merit,
5. Wife has no authority to summon any body since it is the power of the Court and Court will not summon your GF on your wife's allegation of affair based on the chat record only,
6. DV case is filed against the family members who committed domestic violence and divorce case is filed against husband/wife and not against any other person,
7.Illegally collected document can not be accepted as evidence by the Court,
8. Yes, as suggested above you can file complaint under I.T Act for her said cyber crime.