"If it is best not to list things like a period of solo practice on a resume, then what should go on the resume in its place? If anything, it should not appear like a gap in employment history."
So you always need to list things on your resume. There's nothing wrong with listing a period of solo work on your resume. The problem is you want to be very careful about looking too independent on your resume. And that's just something that people typically are sometimes nervous about. If you look like you're too independent and a law firm gets a sense you might become a solo practitioner again, then that can be a problem.