If I were Michael Flynn's lawyer and he were subpoenaed to Congress, what would I do? Have him do exactly what was done by his lawyer - exercise his 5th Amendment right against self-incrimination. Doing anything else would be legal malpractice in my view unless there were a solid immunity agreement. Even the request for documents is a problem since there are testimonial issues in identifying and producing documents that relate to his 5th Amendment rights.
First, don't let a client testify or be interviewed when there's a criminal investigation. Simple. Don't care if he looks guilty. Gotta do it. If you don't it's malpractice or folly. You cannot care about the "optics" or that "someone will look guilty." Clients have a hard time understanding it but it is critical to not give into that type of thinking. I would lay down in the street outside my office in downtown Los Angeles before I'd let a client testify under these circumstances.
I would have Mr. Flynn exercise his 5th Amendment rights for numerous reasons. Let us just use his public FARA filings as one basis since we can review them. To look at the issue, just take a look at the Mr. Flynn's Supplemental Foreign Agent Registration Act (FARA) filings on 3/7/17. They were signed by him electronically under penalty of perjury. Read them if you like [here's the FARA document search link], type in Flynn, click and see what you think.