too_long,
Five years is just way to long. I think you have a good case and will get approval almost as soon as you file the case. I have known of atleast 3-4 people who filed a lawsuit against the USCIS and as soon as the USCIS received the notice of "Intent to sue" they approved those cases at lightning speeds. This is not to say that your approval is guaranteed but at this point a lawsuit seems like the most logical thing for you.
Since 9/11, the USCIS has been using "Security Clearance" as a blanket reply for any and every inquiry. Infact, if they mess up an application and find out that they screwed up, they always inform the applicant that the case is pending "Security Checks". They get away with a lot with this excuse. Last year I had filed for my AP in Jan and when I did not receive an approval till July-August, I started inquiring about the status. At first I got the standard, we will look into the case and get back to you. When I and my lawyer pestered them through AILA, they wrote an email to us stating that our cases are pending "Security Checks" and that constant inquires will only delay the case even more. That was the last straw. I kept inquiring and FINALLY they admitted that they had LOST my AP. Can you believe that? Where the hell did "Security check" come into the picture when they had actually lost the application. I had to refile and got approved within a month of refiling. So do not blindly believe this "Pending Security Check" reply from USCIS, especially since its been over 5 years.
Consult a good lawyer and go ahead with the lawsuit. I am almost certain that you will get approved soon after.
regards,
saras76