请问小白兔及诸位大侠, NIW DIY

1. May I have my petition cover latter written by my supervisor or only can be myself (or an attony)?
2.I am currently a postdoc in the field of biomedical research. How should I address the issue that "national interest would be adversely affected if a LC were required"? beside the all other reasons, may my supervisor emphasizes that LC waiver may give me more opportunities to work in the future in some other lab which is important for my career?
3. Does anyone know any attony or someone can have a prove-reading for my package and of course I'll pay a reasonable charge?
Thanks a lot.