gt1

And now a pandemic to deal with!! But GREAT that this nightmare wait is finally drawing to a close.

Congratulations! They probably have more time to do those kinds of things because of canceled interviews etc. No need to worry about the oath ceremony in the mean time until they start scheduling them again.

Three days ago I received two email in a rapid succession:
1. We approved the petition pending review.
2. We completed the review and placed you in the oath queue.
I was beyond surprised. I assumed that USCIS offices were already closed and the petition would be further delayed.
Now that oath queue may be long, given the current circumstances.
 
Our center's processing times are gain 7 to 18 months. Go figure...

Send the letter. Make sure it's certified mail. It only costs a few bucks, but it sends a signal. 
 
Every advise I've given is what I did personally. In fact, I was even more aggressive. First, I sent them a message every 2 weeks after my initial interview via the USCIS website (same message, copy & paste, with just the timelines changed). They responded by saying the Field Office would be notified. On Day 90 I sent them the letter. Certified mail. I also contacted my Congress rep, and began to prepare to file a lawsuit on Day 121. 2 weeks later I got a call from the USCIS officer. Case resolved.
 
The common error people make is to treat USCIS like a person. They're a bureaucracy, and they're swamped. If your case slips by for one reason or another, you could experience severe delays. It's not personal. You need to know the law/policy, and then force them to act when the law is on your side.

And on lawyer, beside the cost: Even if you end up suing, you must show the court that you've exhausted all your administrative options. You've contacted USCIS by phone; you've gone for InfoPass or it's not an option available in your field office; you've written to USCIS; you've tried going through your Congress rep/Ombudsman. Only then would you have good standing to sue.

They're just blowing you off. And that agent is factually wrong about both the processing time (doesn't apply once the interview has happened) and the N-652 (it's not discretionary). Per the USCIS Policy Manual:
 
I have advised you on how to proceed in an earlier post. Ball is in your court. 

OK. That's problematic. You have no real proof of the decision, then. Who knows if you misheard the officer?
 
No worry, though. If I were you, I would write the Field Officer and ask for an update (check the USCIS website for the office mailing address, and send via certified mail). Ask for an update, recounting your application history (filing date, biometric date, interview date, and interview outcome, and no RFE). Say it's now gone past 90 days.

Okay. It's a single sheet of paper that the officer should've handed to you immediately after the interview. The outcome -- recommended for approval -- would've be hand-ticked. 

I think you should call them up and inquire about your case. I actually called 30 days after interview just to ask if everything is still OK with my case. They were able to tell me that they didn't see any flags, to just wait and to call them up again if I reach the 90 day mark. Fortunately for me, I only had to wait another two weeks or so. You should call especially since your last status is still 'We scheduled...'

Scheduling the oath ceremony takes longer in case of name change.
My interview was June 12 and still no date scheduled for the oath.  I also requested a name change.