FlowerLA

Well, she might also see it like this: how much have you already spent on your friends? How much will you continue to spend on them and how much have they paid back? You're going into a marriage and she might be worried about that too, not just immigration. 

Do not use them. I did and I ended up getting screwed with longer wait time due to them not filing all my evidence in which I ended up getting a RFE. Needless to say what was originally a 6 month wait turned into 13 months. Filing instructions are on the USCIS site for free along with the forms you need. Save yourself the grief and file yourself.

Just do it yourself unless your case has any complications (divorces, children immigrating, criminal records etc). The process is very straightforward, you just need to read everything related to your visa type on the USCIS website and if you have any problems, the forum has most of the issues answered already or you can post questions if you still can't find the answers  Good luck!

As @Roel states your profession has nothing to do with your trustworthiness in terms of immigration.

It might or might not have impact. You won't know until your interview probably. Also your professional doesn't make you 100% legit in the eyes of the embassy. 

Then get your phone records, letters and any other records you have of you both. They need proof of a on going relationship, the best that you have and yes, they want to know how you met. I sent skype, emails, FB, viber and QQ records, that's what we used