You can sue with no problem even the Attorney General, because he is in charge to grant you citizenship. But this is not necessary, you will need to send him a set of summons+complaint anyway.
But in my opinion, you should sue the FBI director. I saw a case, when Plaintiff didn't sue the FBI; USCIS basically said that the name check is stuck at FBI, it is not their fault; the judge bought this and said in the opinion & order, that he can't compel FBI to complete the name check, because FBI is not a defendant in the lawsuit.
I respectfully disagree with you when you say that the law is clear. Unfortunately, in my opinion, this whole mess around the name check is created exactly because the law is not sufficiently clear. Let me explain why I think so:
Congress mandated USCIS to perform a "full criminal background check" on each applicant before they can adjudicate the application. This is sneeked in an Appropriation Act for FY98. There is no clear definition, what is a "full criminal background check", who does it, in what timeframe should be done, and what are the consequences if it is not done in due time.
Apparently, up to November 2002, the name check was done simply running your name on the 'main" file database, by a computer program, and the whole process was done in 1-2 days after the tape was submitted to FBI from USCIS. After Nov. 2002, they decided to extend the name check and include the "reference" file searches and all kind of combinations with your first, last, middle name, with different spelling, different DOB etc. You can imagine, how the number of "hits" skyrocketed. Now if somebody got a "hit" an agent, not a computer program, has to manually investigate that hit and determine if there is real derogatory info about the applicant or it is only a "false alarm", like in the vast majority of the cases. Because USCIS pays only $4/ name check request, FBI doesn't have the necessary resourses to work on these manual processing of the flagged cases. And this change in their name check policy is still covered by the same Congressional mandate, from the FY98 Appropriation Act.
I'm not advocating against the name check. This is probably a very important tool to screen out criminals, potential terrorrists, spies and all kind of bad people. They should do this, but in a more efficient way. DHS and FBI would need to go back to Congress and ask to ammend the law to define precisely what should contain a "full criminal background check", who should do it, in what timeframe, what happens if is not done in the mandated timeframe and who pays for the cost. If they need to raise the cost what USCIS is paying to FBI from $4/request to $40/request or $100/request, it's fine. I'm sure that we all agree that we would rather pay $36 or $96 extra in the application fee, than wait for years and pay a lot more after for the lawsuit to force them to act.
The rollercoaster evolution of your mood is normal. I think that we all went through this and you should remember that you will ultimately prevail if you are stron enough and have patience. Failure is not an option. We are here to support each other and first of all, to help each other with (preferably) reliable information. Knowledge is power. (I know a lot more of such slogans, but I will abstain from them in the future
).