My last face to face with the DOL they sent two people and they tried a 'good cop, bad cop' routine. They tried every trick in the book to get me to say something wrong. The 'bad cop' ended up terribly frustrated.
So yes, get an attorney. This is an investigation and not an audit.
Who is your 5500 software vendor? Perhaps this is a problem related to their specific software? I haven't heard of this being an issue before. Have you checked with them?
Yes, I did check that the employer name changed. The error message is referencing the plan name being different though. No place to check for that......