John Fulton Reynolds accepts command of the Army of The Potomac

So i assume John Reynolds does not die at the start of the Battle of Gettysburg. 

Would he be a rival to Ulysses S. Grant.

So reading this, American Battlefield Trust segment on John F. Reynolds:

After the Battle of Chancellorsville, Reynolds was one of several Union Generals who urged Hooker’s removal from command. However, when President Lincoln met with Reynolds and offered him command of the Army of the Potomac, Reynolds replied that he would only accept the command if he could be sure there would be no interference from his superiors in Washington. Unwilling to sacrifice civilian control of the military, Lincoln instead put Reynolds's friend General George Meade in command.

Does that mean your point on him not wanted to work for Halleck is true as well.

Getting back to the destruction of the Army of Northern Virginia. I'd think the Army of the Potomac, despite the weather could hound Lee as he moved south toward the Potomac. Logistics would be a key point. The most vulnerable part of the ANV was it's supply & hospital trains. Lee would probably be forced to abandon his wounded to the care of the AOP, but he needed those supplies. No way for him to replenish as long as the AOP was close on his heels.

Lee's only counter was to rely on his Cavalry screen. So that would be an early object of Reynolds. Somewhere along that march Stewart's cavalry would have to be brought to battle as it tried to screen the supply train from the advancing Union forces. Stewart's Cavalry, like the rest of the ANV, had been savaged at Gettysburg. The Union cavalry had also suffered losses but it was well able to make good those losses in short order. The ANV in enemy territory could not. So the clash would happen and even if Stewart pulled off a victory his already depleted force would be spent while the Union cavalry would be replaced by fresh regiments.

Once the ANV cavalry was used up Lee would have to make a very hard decision. Did he have the supplies for another good fight? If so, he would dig in on favorable ground and pray the AOP would conduct a frontal assault.
However, they were still in enemy territory and Reynolds need do no more than hold in place and let the ANV starve. So I'd think Lee would continue south as fast as he could go.

Reynold's objective was still the destruction of the ANV. He would use the hammer and anvil. The USN would move gunboats up the Potomac and coordinate with either a flying column of infantry or a large cavalry force to deny Lee a crossing of the Potomac. The AOP would hammer the ANV against the anvil of gunboats and troops. Result the destruction of the South's most powerful army and the loss of invaluable senior officers and veteran troops. Who knows, Lee might have had another heart attack.

So in my ATL the ANV is annihilated. Lee is dead and with him most of his senior surviving commanders.

Now what does Jefferson Davis do? He swallows crow and begs Joe Johnston to somehow put together an army to defend Richmond. No other surviving confederate commander is a near equal, maybe even equal or better than Lee than Joe.
As I see it given the loss of the ANV Joe would be forced into fixed trenches defense of Richmond. No way a scratch force could be expected to maneuver against the Veterans of the AOP. AOP would now adopt siege tactics. This would take a while all depending again on logistics, southern logistics.

I could see Sherman being turned loose on the interior of the confederacy as he was in the OTL and he would still make the South Howl. With luck it is all over by Christmas 1863. Without luck early 1864.

without Lee to tell his men to go home, I fear the aftermath would be a prolonged guerrilla war extending more than a decade. I think Joe Johnston would make the same appeal to his men but despite Joe being a great general the troops did not love him the way they did "Massah" Robert.

The only real hope is that Abe Lincoln could reconcile the south by his policy of "Letting them up easy". The assassination of Lincoln would mean the occupation contending with a widespread insurrection would begat atrocity answered with reciprocating atrocity and the 'Freed" slaves in the middle of it. All I can see is an ongoing blood bath.

I think I need a single malt and a good cigar.

Watching this good tour of a Civil War battlefield and reading this article: A Civil Ending?, it would not help the South get as the article says:

Lee's much feared veteran force, still posed a credible threat to Union foes. To surrender under those circumstances, as a viable, united nation rather than as a fragmented, conquered rabble, might bring a more lenient peace, spare the South absolute subjugation and persuade history to judge the Confederate cause kindly. A gracious, voluntary surrender, Breckinridge insisted, would yield an honorable defeat.

But of course president. Jefferson Davis insisted that victory alone could save the South, and he meant to achieve it, even if he had to disperse his armies and wage a guerrilla war. His unrealistic predictions of ultimate victory bordered on the ''delusional,'' William C. Davis says, but for Jefferson Davis, an honorable defeat -- if, indeed, defeat must come -- meant going down swinging.

Well as always, you see politicians trying to be politicians and soldiers trying to be soldiers.

I like watching the American Battlefield Trust, they are fasianteing to watch.

But back to your thread. 

Destroying the Army of Northern Virginia in 1863/64 after the battle of Gettysburg might shorten the war, also this might butterfly away the death off one of my 3 greatest US presidents.