Your defense of primary sources is well argued. It can be difficult to explain the validity of primary sources, especially to people without formal training in history.

Mosby deserves consideration as a soldier who knew Stuart and a participant in mounted operations in northern Virginia. However, Mosby didn't ride with Stuart during the Gettysburg Campaign, and wasn't involved in the planning for the campaign. Mosby's opinions and observations have weight because he's writing as a veteran and someone who knew Stuart, but he doesn't have great insight into operational plans for Lee's movements north of the Potomac. Anyone could read the reports and correspondence as Mosby did.

It's an older book, but Clifford Dowdey's "Death of a Nation: The Story of Lee and His Men at Gettysburg" explains mistakes made by each of Lee's senior officers at Gettysburg. Dowdey provides background on articles by James Longstreet, who offered three different versions of Gettysburg.
http://www.civilwarstandard.com/product_info.po%3bjsessionid=14oxnUBM2KPN-mYyoohreQYH?ID=2269

Dowdey makes it clear that no one officer should bear the blame for defeat at Gettysburg.