advice needed for my parents' B-2 visa application

Originally I came to the States to attend a business meeting (via B-1 visa). Afterwards, i decided to stay to pursue my master degree. So I changed my B-1 visa to F-1. After graduation, I found a job here. Later on I got my green card. And now I'm a US citizen. I'm planning to invite my parents to come to visit me here. My concern is that whether my visa history would have any negative impact to them. Let's say what if they will be asked by the question like how did your daughter go to US or what kind of visa did your daughter get when she went to US. Should they answer the truth?

Another question is about my father's employer, he is a doctor works for 化工职业病防治院, after translate that name into english, it becomes somthing like "occupational disease prevention center of Chemical Industry". I know US is quite sensitive about anything related to chemical now. When we fill DS-156/157 form, should we just write 职业病防治院/occupational disease prevention center in his employer field on the form(remove Chemical Industry from his employer name)? Or we should fill his employer actual name?

One more question, although i sent/told my parents many interview preparation tips I got from this website, I'm still a little worried about their visa interviews. Because neither of my parents are eloquent talking persons, would it be a good idea to hire a visa service agency to help them prepare for their visa interview?

Your help will be very much appreciates.