Do you have the first notice with the 'no decision can be made' with you? If so, can you please read the last paragraph of that document again (and then, read it again). USCIS can't keep resetting the clock by calling you in for your 'next' interview.
Take it from one of the people who had to wait 3+ years for security clearances - Infopass doesn't mean anything. Your only reason to go to the Infopass is to get USCIS to say something really silly so you can add it to your 'complaint'. In your case, USCIS told you it could take 6 more months to get a decision to you while the piece of paper they handed you after your first interview said that after 120 days you can ask a judge to intervene.
What are you going to do if you don't hear from USCIS for another 6 months (or worse still, you get a 3rd interview call a month from now). Is your lawyer going to tell you to go for that interview and then wait another 120 days or are you going to file a 1447(b) action with a pending interview. (If your current situation isn't 'ripe' for a lawsuit, a pending interview is only going to make the situation worse as in that case, the Judge will try to let USCIS decide your case before intervening)
(But i totally understand your thought process... I too waited 3 years while others filed complaints after 18 months ).
You don't need a lawyer for this. You can file this Pro Se (for yourself) and should get this sorted pretty quickly.
if your case is clear of any issues, go ahead and send them an advance draft copy of the complaint you intend to file.
That should get the case moving.