Anyhow, one of the keys to the PCL, I think, is the extensive road network that would allow Meade to shuffle troops from one threatened area to another — just as he did at G-burg. He might have been stretched, but I think he could have met the demands. He really would have no choice with Baltimore and Washington DC not all that far away. I still think the PCL would be too strong for Lee to crack. He'd be like a wave crashing against rocks.
I think he probably could, and you are right that the position is a very strong one for defense. The one projected, though, covered about 20 miles with 7 full-strength corps. If Meade is to pull back here after July 2 or 3, he may not have enough troops to hold it all.
In that case, he has to make choices. If he is going to cheat somewhere, it will probably be on the eastern flank, towards Baltimore. Coming from Gettysburg, it is easier for Lee to strike at Taneytown, and so Meade would have to lean his strength that way.
Because of the strength of the position, the real danger is that Lee will concentrate most of his strength against a single point to overwhelm it. Lee will also be reduced in strength, and so cannot spread his force enough to threaten multiple points along the line.
You know, I thought about the South Mountain option, and I think it's a creditable one. But Vicksburg has just fallen, and having Lee on the west side of South Mountain would be intolerable. Could not some of those western Vicksburg troops be released to pressure Lee from the west and force the issue?
That's the only real source of veteran troops, but it will take a few weeks to get them up to central PA. Grant returned Burnside's troops after Vicksburg fell, but it was August before they were up and ready to participate in the attack towards Knoxville. Troops from Grant have to come up the Mississippi and maybe the Ohio to get to some point where they can get on a RR.
They might decide to grab a division or two from Rosecrans after Tullahoma. Coming on the heels of his success, that would be difficult, and Rosecrans might have been the "best" in the war at digging in his heels and delaying what his superiors wanted done. Sherman loaned A. J. Smith's Corps to Rosecrans in Missouri in August of 1864, and it was supposed to be sent back in the first half of September. It arrives in Nashville for Thomas around November 30, and Grant finally had to order Smith to ignore all further orders from Rosecrans to get it done.
That means that, realistically, there are no troops from Burnside/Grant/Rosecrans available in middle PA until late July. If the Union is going to attack Lee behind South Mountain, they are going to use the forces already in the East: Meade's AoP, Dix's troops, the WV forces, the militia around Harrisburg, and probably a bunch of new regiments drawn from wherever they could lay hands on them.
If Lee has his army together (Stuart has rejoined), that's a very tough task. Attacking over the mountain through the gaps will be ugly. Coming around through the north end of the Cumberland Valley will be time consuming and problematic. Not enough force to the West to press Lee. Any force trying to cut Lee's LOC is vulnerable to Lee turning on them so quickly they cannot escape.
I doubt Lee could have stayed until September, as he mentioned. I doubt the Union could have driven him out before late July at best if he had stood on the defensive.
Tim