As the other attorneys have stated, the interview is a part of the petition process and should not be a cause for concern. I have never been involved in order heard of a case which did not require an interview. In some cases, there can even be two separate interviews (one to determine the validity of the marriage and one for adjustment of status), although I have never experienced such a case.
The focus should not be to attempt to avoid the interview, but to be prepared for your interview.
First, I am very sorry for your friend's circumstances. Leaving an abuser is incredibly difficult and very dangerous. She needs the support of everyone in her life- including her family and friends.
Given the facts you've provided, your friend should be able to protect herself without having to leave her job. Her employer should have a copy of all restraining orders and she should call the police anytime he is in violation of those orders. She might also want to notify her employer of her...
I agree with my fellow attorneys. Although you may technically be able to travel from state to state, there is definitely a real risk involved where you're stopping in places that you could be subject to inspection (airports, etc.). And, it's a risk you have to be very aware of and decide if it is worthwhile to travel when the consequences could be deportation. I generally recommend against it, but it's your choice.
You don't have to be a lawyer to prepare these documents, but having a non-lawyer help you do so can be dangerous.
There is no accountability or ethical obligation and often no assurance the work is even being done- let alone being done correctly. These waivers are difficult, so the assistance you seek should be from a trusted professional.
Also, there are many scams involving non-lawyers providing "legal assistance" for excessive fees and with horrible results, check out the USCIS website...
Is your fiance a US citizen? If he is, you should can marry abroad and then he can petition on your behalf. With respect to sponsorship, that is a financial question and more information is required. There is an extreme hardship waiver that will be needed if your fiance is to have any success in bringing you back to the US. That waiver makes the process more expensive and more complicated. You and/or your fiance should schedule a consultation with an attorney so that you can more fully discuss...
A fraudulent marriage, simply put, is one entered in to for the purpose of obtaining immigration benefits. In deciding whether that has happened, the government will look at a lot of things- like whether the "couple" lived together, filed taxes together and otherwise joined their lives. They'll look at whatever they need to in order to make that determination.
I'm really sorry to hear what your cousin has been through. I hope he's doing well. Unfortunately, I think more information is needed to evaluate your cousin's prospective case. He should get a free consultation from an attorney with a good amount of U visa experience. Many attorneys, myself included, can do the consultation over the phone. All it'll cost is time.
It is always the highest priority to be truthful with the government regarding all requested information, so you need to disclose the information. It is true that the government will "forgive" your unlawful employment, but that's not a reason not to disclose it. Quite the opposite.
Also, make sure you've paid taxes. The unlawful employment is one thing, but failure to pay taxes is quite another.