The two reasons that you think caused your application to be returned can be fatal. Think about it. To migrate under FSW, you need to be eligible under a nominated occupation. How does CIC know that you're eligible? It cross references the service letter that you provide against duties normally associated with that occupation as described in the NOC. If your job duties only match 20 to 30 percent as you claim, then CIC has a valid reason to reject your application and give a PER to some other applicant who has performed duties that closely match the eligible occupation's duties.
You mention that this time you're submitting a reference letter on a PLAIN SHEET OF PAPER! I don't think this is advisable. On the document checklist (section 12), it specifically says that the employment letter must be on a company letter and have the relevant contact details (telephone, email, etc). So, it's best if you follow instructions to the letter and get your employment letter on a company letterhead. If you have to, go to HR and plead with them to write your duties on a company letter head.
Birth certificate is a very important document. It is one of the primary documents through which CIC establishes your age. It takes added significance, especially since points are awarded for age. I'm not familiar with how things work in India, but can't you apply for and get a new birth certificate WITH YOUR NAME ON IT? If you can, I very strongly suggest you to do it as a birth certificate with your name will greatly lessen the chances of problems with this crucial document.