You need to take a look at why the Conf. Army was in Penn, RfR.
There was no strategic reason for a battle to be fought at Gettysberg. At Longstreet pushed for, rightly, the better call would have been to position the Army between the AotP and Washington DC, however, that could have left them open to a flank attack by defensive troops in the DC area. If he had moved south, then he'd be traveling along the flank of the union army. If he had moved north, he'd be stretching his lines further north of his supply base- and a possible attack by union troops while thusly stread out.
However, Lee took a risk since the AotP had a new commander, and not knowing all the disposions of the Union troops, that he could bat them aside and continue to march.
Lee had to get out of the South in order to keep the enemy off the lands long enough to get a good crop into the ground. Also, you have to keep in mind the 'peace' party in the north would use this invasion as a plank to unseat Lincoln. If Lee could defeat the AotP, or even fight them to a draw- he'd win.
Supplies were a factor in moving to Gettysberg, although not the most important one. The road junctions simply served to bring all the various Corps to the battle quicker. Seazing them didn't do anything to help Lee's plan. It would have tied him to a geographical objective- rather than the political objective of the campaign.
Lee’s reasons for invading the North were political, military, and
economic. Politically, the prospect of European intervention on the side
of the Confederacy would be greatly enhanced by a decisive victory on
northern soil. The military objective was the capture of Harrisburg, the
capital of Pennsylvania. With Harrisburg in his hands, Lee could threaten
Philadelphia, Baltimore, or Washington as circumstances might make
advisable, and he could also cut the Pennsylvania Railroad, a vital supply
line for the Union armies. Such a campaign was a sound if bold concept,
particularly as Lee counted on outmarching the Army of the Potomac
and meeting with no opposition except that of militia.
The economic reason for the campaign had to do with such mundane
things as food, forage, horses, shoes—in fact almost everything an
army needs except ammunition, with which the Confederates were well
supplied. The Confederate commissary system, never good, had so broken
down that the army had no alternative but to “live off the countryâ€â€”
not in the friendly Shenandoah Valley, but in the hostile Hagerstown
and Cumberland Valleys.As for your two questions;
Would have Stonewall made a difference?
Probably not. Although if he had been leading the lead divisions odds are he would have pushed on and taken the 'high-ground', that would still mean, what? The AotP would have still came on, and simply positioned themselves further east in defensive positions. Lee would have still been forced to attack- or wait in position and been flanked. Pickett's Charge may have happened in a different location.
If Lee would have moved south, more south east really, what would have happened?
http://www.dcnr.state.pa.us/topogeo/education/es5/es5.pdfMeade, however, did not want to fight at Gettysburg, desiring a
stronger position. The line which he selected, generally known as the
Pipe Creek or Westminster line, might be better described as the Parrs
Ridge line. Parrs Ridge, in the western edge of the Piedmont, extends
northeast and southwest through Westminster. It forms the divide between
the Monocacy River drainage on the west and the direct drainage
to the Chesapeake Bay on the east. The ridge near the Pennsylvania-
Maryland State line stands at more than 1,000 feet above sea level, and
at Westminster about 800 feet; this compares with the usual Piedmont
elevations of 400 to 500 feet. Pipe Creek, flowing through the Triassic
basins north of the ridge and into the Monocacy River, is not particularly
formidable, but Parrs Ridge, to the east, upheld by highly resistant
schists and quartzites, has not only height, but widths of 4 to 10 miles that
could have been fortified into an almost impregnable defensive position