my opinion on your queries are as under:
1. At what stage, I have to submit my evidence and how?
Opinion: Petitioner evidence can be filed after framing the isssue.
2. If She refuses, it is not her hand writing, how to defend my case.
Opinion: There is a provision in Law related to hand-writing expert's opinion
3. Is it necessary to submit original to the court.
Opinion: if you have original then original will be better to file before the court
4. What about submitting Copy of letter certified by Notary Public (One Notary Public Refused to acknowledge the copy saying that court would ask his presence and it would be unnecessary trouble for him
Opinion: notarized documents can not be parlel as original so file original if you have
5. I fear for the safety of the letter because it is the only vital piece of evidence in my possession. How to protect the same.
Opinion: file before the court & get certified copy of the same.
6. Few of email has been received in my company email account. I have converted in to PDF file before leaving the company.
a. Whether that PDF is accepted as a Proof. Yes it may be
b. I have forwarded few mails received in my company ID to my bro’s ID which I have received back in my personal ID. It is acceptable as evidence?
Opinion: yes these email's are acceptable as evidence as per section 65B of Indian evidence act