The ITR has your PAN. So that should not be an issue.
Your letter of explanation should convey:
1. Why do you want to pursue the program specified in your LOA
2. How does your educational background career qualify you for the program
3. What personal/professional goals/benefits do you intend to achieve through this program
4. How do you plan to fund your studies (entire duration)
5. What do you plan to do after returning to your home country, after your studies
Restrict these to a single page or single and a half.
You may or may not explain the difference in surname. It is trivial.
Do create an index of documents you are attaching to the LoE.