Case review and interview is all discretion based. If the officer thinks that there isn’t enough evidence, he can issue an RFE or deny your application.
If you apply for the i485 before getting FR, you are ineligible. But they receive over 800000 adjustment of status applications all over the country. Some of these officers aren’t experienced enough with the waiver process and they check with their supervisors. Some offices might go by the book and deny it, some will issue an RFE, and sometimes it just slips and goes to the interview.
At the interview, I guess it would be rare that an officer does what the lawyer told u, but even if he did that. U can go to administrative appeal and they will approve it. I saw somebody’s post on a Facebook group about an interview. The officer asked for the waiver and she just showed them her visa and DS2019 saying not subject and she got approved.
It’s a risk to send the i485 before the waiver. You might get lucky and all come together at a perfect time. You get an FR from DOS and you get it before an RFE or interview.