You don't need HR for petition letter.

I got the Petition letter only from the Dean of my engineering school and received employment letter from HR in my case. You may have different.

Nobody knows exactly the difference between your major and your job duty. You have stronger background if you work in wider areas. You can mess them up but be sure to demonstrate you have internation reputation in the area you're working on by showing that your papers are published in the top journals in your field, your presentation is in the audience of International Conferences.etc.

By the way, you have more chance if your boss can change your title of research associate to Research Scientist or Research Engineer, or others you feel better since the EB1B is in the category of Outstanding Researcher/Professor.

I think you have enough evidence to try EB1B.

Good luck!