I wanted to provide an update on the visa interview my parents had two weeks ago. We had all documents and I had no doubt everything will be ok. My dad received an approval on the spot and my mom was refused a visa due to... A LACK OF MAIDEN NAME ON HER POLICE CERTIFICATE FROM BELARUS. This was so frustrating as nobody ever told us the maiden name had to be included (though it is logical). So if you have a fiancé/mom/sister applying for the US immigrant visa in Warsaw, please ensure the maiden name is covered in the police certificate (again, this information is nowhere to be found, so just include the maiden name!). Otherwise you will get a green slip from the US consular officer and you will have to go back to Belarus to obtain the new certificate. Luckily, you can order such certificate in Belarus within 3 business days, but the travel back and forth is draining as now it takes up to 24 hours to get to Warsaw taking into account all the border controls, plus you have to pay for additional nights at hotels, have to wait a lot, etc... My mom told me there were at least 10 women in line with her with the same issue as she came back to Warsaw, so this seems to be a recurring issue for many Belarusian women applying for the immigrant visa.
Another thing to keep in mind - if you are refused and were given a green slip, this is not "Administrative Processing" which may usually take months (and I think you would have received a white slip instead). The difference between a green slip and a white slip is that the green slip means your visa is on hold and US consular officer requires some additional information (so I believe this means your visa is approved pending additional documents), and so my mom had to bring the new police certificate. Your CEAC status will state "Refused" but once you bring the missing document(s), your visa will be approved and my mom's CEAC status quickly changed to "Issued" and she received her passport with visa in the afternoon of next day following her submission of the new certificate. The white slip and status on CEAC "Administrative Processing" means they need to dig deeper in your case and you may be asked to provide more documents and information upon request, and this usually takes months from what I read..
Overall, the interview process in Warsaw was smooth and took only a few minutes. As long as you have all documents in order and uploaded in CEAC, you should be fine. If you have questions, you can always call the Polish or US numbers and there are very nice people working there who will answer all your questions.
I hope this information is helpful. If anyone has any additional questions, please let me know.