The problem with Longstreet's plan is that it wasn't particularly well thought out. He wants to move south and get around the AotP and my questions are "How?" and "Where to?". The only road immediately south potentially usable was the Emmittsburg Road which doesn't lead in the right direction and using that road assumes that the AotP stays put and doesn't use the Taneytown Road to move itself between Washington and the AoNV. The only other option is to move west, passing South Mountain and, again, that is going in the wrong direction and assuming the AotP isn't going to move to interpose itself.
And none of that discusses moving the army in the face of a prepared enemy, which is seriously problematic at best.
R