If you need an attorney somethings wrong.
Many years ago, when third parties were allowed in interviews, I had an IRS union rep sit in on a Subject Interview. The rep kept interrupting and interjecting, I gave two warnings that I was going to end the interview due to the disruption. Wasn’t five minutes later, I was interrupted again. I packed up my stuff and left, much to Subject’s dismay.
Smart people don’t base their actions on what may look bad, they serve their own self interests like everyone else in DC. The investigator serves the interests of the government, and their life gets easier and the rules more fungible when nobody is serving the interests of the person being interrogated. (No, it’s not an interview.) If as a previous investigator stated they report only the facts, and investigators leave their biases out of reports, it should make no difference if a lawyer is present or not because if they have a bias against people bringing an attorney that should not affect the end result of the report. But you have just made a defacto admission this is not true.
How about this: save your money for the appeal. Hopefully there won’t be one, but it makes more sense to have an attorney help you with any appeal than an interview. But make sure it is an attorney with some experience in security clearance appeals.