Your case WILL be set for an interview. Every waiver of joint filing is required by USCIS operating procedures to have an interview.
So you can safely go and return as long as someone is monitoring your mail and will be able to open anything from USCIS, scan and email it to you so you can get back here in time for the interview.
The case will not be denied without an interview. I believe your case will be approved since your ex wrote that leter and you had joint insurance, taxes, health insurance and affidavits from friends and neighbors.
However, you will still be called for an interview.
You cannot miss that appointment or your status will be terminated.
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