So nobody knows anything huh?
Off the top of my head, the Battle of Glendale could not be the Cannae that the Confederates were hoping for, at worst the Army of the Potomac would lose a significant amount of strength but not complete destruction. Jackson was not striking an open flank, he would have to assault through the VI Corps (though it was rather unprepared for a fight at the time). In addition, W. F. Smith has two of Sedgwick’s brigades available as a reserve, so it wouldn’t exactly be an easy fight for Jackson to sweep across the Union flank.
Another issue for the Confederates is that Longstreet had yet to grasp the importance of the mass and coordination of an assault. Longstreet’s assault was effectively piecemeal, ruining any chance of a decisive breakthrough at the Union centre.
Holmes’ division had no chnce against Porter’s Corps which were aided by gunboats and Huger’s division is effectively unable to reach the field due to a number of reasons. To make things worse, Magruder basically played d’Erlon during Quatres Brad and Ligny, spending the day marching and countermarching.
Basically, how well Jackson does in his attack is important as to how successful the Confederates are. If he can push back the VI Corps, along with Richardson and Sedgwick’s brigade, the Union line might have to be stripped to halt Jackson’s assault. However, Longstreet’s uncoordinated assault still diminishes the chance of a decisive Confederate victory. I feel that at best the Confederates could have force or even rout the Army of the Potomac’s rearguard with significant losses, but it simply would not have resulted in a Cannae.