I-751 Still Pending For Interview

I had already submitted my wif’s paperwork back in November or December of 2018, got the biometrics done in January 2019, then the status changed to your case is ready to be scheduled for an interview in April 2019. It is still on this same status, and she has been here in the US for three years. Should we go ahead and start her N-400? Also, should we call the USCIS? Finally, our case is with the national benefits center based on the USCIS case number we got that begins with MSC, yet when will they usually update if they need us to interview or if they feel everything is sufficient and to give her the green card?