If Lee was ill, and unable to command, he had an obligation to turn the command over to an officer capable of conducting the battle. That would have probably been Longstreet. Lee chose not to do this. He, and he alone is responsible for the debacle at Gettysburg. A fact that the lost causers refuse to recognize.
What the "Lost Causers" (a movement started by Jubal Early, one of Ewell's division commanders at Gettysburg) don't seem to realise is that even a Confederate victory at Gettysburg would have done nothing for the continued existence of the Confederacy. And if the 15th Alabama manages to roll up Chamberlain and the 20th Maine, it is no gaurantee that Law (who took command of Hood's division) would have sent support to the 15th. Perhaps Robertson's brigade would have, but they were pretty tied up on their own. It is no sure thing that they would have rolled up the Union left flank. Meade had good interior lines, and two relatively un-engaged corps, XII and VI corps (which was just arriving, but could have been used in an emergency). Two divisions of V corps were engaged...Vincent's brigade on Little Round Top was in Barnes Division and the division was deployed where Sickle's III corps should have been. Ayers Division fought in the Wheat field. Crawford's Division was not engaged until after the serious fighting of July 2 was over. They could have easily plugged the hole on the left had it been opened.
Back to point though, Gettysburg would have essentially bought nothing for Lee. He simply could not have continued the campaign because of logistical considerations. To do so would have cut him off from his supply lines and made the situation worse.
Bottom line, the North held a 3:2 advantage in available manpower, and an approximate 7:1 advantage in production of war materials. No way the South could have won. The best hope would have been a negotiated settlement. That wasn't going to happen under Lincoln or McClelland had he won the 1864 election.
i and other history students suspect that what Lee really suffered from during Gettysburg was what is called "The Victory Disease". He had an over inflated sense of his abilities, and the army's abilities after Chancellorsville. The actual record of the ANV during Lee's tenure as Commander doesn't really back the idea that Lee was invincible. Aside from that, up until the time that Grant came east, the Army of the Potomac was tied down by orders from Washington that forbid it to leave Washington or Baltimore uncovered during operations.
Courage aside, the South could have NEVER won. The logistics were simply not in it's favor.