First you should not panick. That definitely doesn't help. You are in a much better situation than many of the forum members, who are still waiting for the FBI name check result. Your interview was less than a month ago, USCIS has legally 120 days to adjudicate your case after the interview. You don't face deportation, you can freely work and travel so there is no imminent danger. Of course, it is nerve wrecking this waiting game, and it is taking a toll on many of us, but I still belive that you will be able to sort this out without filing a lawsuit. This case is a very good candidate, in my opinion, where a polite letter thretening them that you will file a lawsuit if no decision is made on your application after the legal period passed, most likely will have an effect. USCIS knows that they most likely will lose such a lawsuit, they can't even blame FBI for not completing your name check.
It is difficult to answer your question about filing a lawsuit by yourself vs. hiring an attorney. This really depends how much time are you willing to dedicate for the preparations to educate yourself in these legal issues. There are many forum members who did this process by themselves, and won their cases, so it is certainly possible. You "just" need enough time and patience to sit down in front of your computer monitor and start reading the postings on this forum. Practically all the info necessary to fight successfully your case was already posted and there are several very knowledgable active forum members, willing to answer questions you may have. Again, because your name check was already completed, your case is much simpler than most of the other stuck cases, where we had to argue that examinaton=interview, and the 120 days from 1447(b) was triggered by the interview and should not be counted only after the name check is completed.
If they really lost your A file, it is certainly their mistake not yours, you can't be penalized for this. I have really serious doubts that any AUSA would have the guts to tell to a federal judge that "your Honor, USCIS lost the Plaintiff's A file and because of this decided to not adjudicate her case forever". If the judge would not make a stoke at the spot, s/he certainly would become really angry and decide the case in your favor.
Bottom line: start reading and prepare yourself for a fight, but keep pressure on the USCIS by going once a week or once every two weeks to an Infopass and ask about your case. When the 120 days passed and still didn't receive your oath letter, you should write a nice letter to your interviewing officer or to Mrs. Carol Jenifer (the Detroit DO director), explaining your situation, your desire to solve this case peacefully but also your firm determination that as a last resort you will file a complaint in the federal district court based on the 8 U.S.C. 1447(b) if you don't receive your oath letter in 30 days from the date of the letter of intent to sue.
If you don't feel comfortable at all with the idea to go Pro Se, we can recomend you some immigration attorneys who will represent you. Lowtechguy (one of the forum members) is using the services of one of them, he is in your district.
