Don't forget that WOM is a court order requesting a govermental agency to perform their job. You need something else besides just the evidence that USCIS and FBI *did not* perform their job. Most likely, you will have to prove that USCIS and FBI *should have* performed their job, which means you must be almost the only one suffering [that much] from their delay. If there are lots of cases that are delayed as well, they have a good case claiming a backlog affecting everybody, so a WOM granted to just one person would be unfair to the rest of the applicants in the queue, while granting WOM to everyone would be unpractical and impossible to perform by the agency.
Additionally, in our capitalist world, it would be helpful to show some actual damages (read, lost money) you've incurred during this delay. These typically must be *actual* damages and not *imaginary* damages, because imaginary damages could not be foreseen by the USCIS. Then, who knows, but the court may award you those damages (payable by USCIS : )) as well as WOM. When I say *actual* damages, I mean you have to prove them - for example, money spent for EAD renewal (except if paid by employer, unless the employer joins you as a plaintiff), actual offer letters you had to reject because of your status (calculate the lost salary). The rejected offers could be hard to prove, though, given the I-485 portability.
And yes, you have to prove that you've excersised all reasonable (and maybe unreasonable) steps on your own, to get the agency moving, and to prove that the agency could have and should have performed their job.
I'm not a lawyer, any of your actions, including refraining from an action are at your own risk, or consult a professional.