1. They have approached court with a petition seeking annulment of marriage for the said reasons.
Once you received the summons from court you have to appear before court and file your counter denying all her allegations and can mention that the burden of proof lies on her.
She has to let in her evidence with documentary evidence or any other substantial evidences to prove her allegations before the court and to present her arguments.
The court after hearing both the sides will deride based on the facts and merits in the case.
2. In annulment case she cannot ask for maintenance or alimony. About filing criminal case under 498a ipc, it is her decision, you cannot prevent her from going ahead with such cases, you can obtain AB and then challenge her false allegations in the trial court.
3. You have to fight back to deny all her allegations otherwise you will be tagged with impotency which may be harmful for your future in case you want to remarry.
You can file a divorce case after one year of marriage on your own but contest this and get it dismissed by all means.
4. Defamation case will be maintainable only after proving her allegations as false and not before that.