It wouldn't look bad at all if you filed the lawsuit now. I've never heard of a "no-process pile", especially considering the fact that all your background checks have cleared a long time ago. Would you happen to have a written document stating that your application was placed in this "no-process pile"? If yes, then go ahead and sue. This would be the ultimate proof that your application is being unreasonably delayed.
As far as the USCIS guaranteeing to your Senator that your application is in queue, I wouldn't take too much stock in it. They are known for their lack of accurate information.
As far as the USCIS guaranteeing to your Senator that your application is in queue, I wouldn't take too much stock in it. They are known for their lack of accurate information.