It couldn't have have brought about a truly military victory in any case -- by that time the Confederacy would not have been able to sustain the sort of offensive necessary to reach DC, for the same reasons you give for them reaching Philadelphia.
Confederate hopes for victory at Gettysburg would have depended on its impact to Union morale, and the war-weariness that would result from the expensive effort that would have been needed to oust them from the Gettysburg area.
I fully agree. Lee did have a third option also and that would have been to do as you suggest, bide his time in Gettysburg, while he allowed his troops some rest, strengthened his lines of supply and sent small foraging parties out to wreak what havoc they could in say a 20-30 mile radius of Gettysburg.