Lee made one big mistake, he was at Gettysburg. When Davis chose not to support Lee’s strategic campaign proposal, marching off into Pennsylvania unsupported was a profound error of judgement.
I don't know if I agree with that. Both Longstreet and Lee were in agreement that something needed to be done and the questions was whether to reinforce, Bragg, Pemberton, or Lee. Clearly Lee is the best of the three. In fact, though many claim Longstreet as a supporter of the "Western Bloc," he actually full-heartedly supported Lee's invasion.
He wrote his ally Senator Wigfall on May 13:
"There is fair prospect of forward movement. We can spare nothing for the Western Armies. On the contrary we should have use of our own and the balance of our armies if we could get them. If we could cross the Potomac with 150,000 men, I think we could demand Lincoln to declare his purpose."
Longstreet also told Wigfall:
"When I agreed with the secretary and yourself about sending troops west, I was under the impression that we would be obliged to be on the defensive here (Virginia)."
The Choice in May of 1863 was either Mississippi or Virginia, and if we follow the timeline, it would have been next to impossible for AoNV Troops to reach either Bragg or Johnston in time to stop the fall of Vicksburg, and in doing so would have weakened Lee's army, which had the best record of beating Union armies before. If Lee's army stays in Virginia, and doesn't invade you are risking the 1864 Overland Campaign in 1863, and Lee would have lost the initiative in losing 2-3 divisions to the West. They also couldn't keep feeding their soldiers where they were at, which was part of the reason Longstreet missed the battle of Chancellorsville. Lastly, the AoP was nearing the end of many of it's regiments 2-year enlistments, and so July onward would give the AoNV the best chance to fight at near parity.
I believe Davis supported the invasion, but in typical Davis fashion, never gave it the troops it needed to be decisive. Davis in a letter to Lee on May 15th wrote to relieve Lee of any concern for Richmond's safety "while you are moving towards the North and West." He also wrote in that same letter "I concur entirely in your views of the importance of aggressive movements by your army." Secretary Seddon also noted that the movement of the AoNV north was "indispensable to our safety and independence."
The issue comes from Davis not being assertive, and clearly not understanding exactly what Lee wanted in his move North specifically, the return of his seasoned troops (Cooke, Ranson, Corse, and Jenkins) roughly 6,500 men, instead he got Pettigrew and Davis. 2 untried brigades in exchange for four veteran units. Not a good exchange. On day 1 you would of had 4 veteran brigades in Heth's division, and while I feel Pettigrew did a decent job on day 1, it's clear that Davis did not. He also exchanged out Colquitt's veteran brigade for Daniel's untried unit, but I think that was more he didn't trust Colquitt.
On top of that Lee wanted Davis to instruct Beauregard to collect all troops not needed near Richmond and the Carolinas and act as a 4th Corps if you will. Lee wrote Davis that it would be better, "to order Beauregard on with all forces that can be spared, and put him in command here, than to keep them there inactive, and this army inefficient from paucity of numbers. Even Longstreet mentions Beauregard in a June 3rd letter to a comrade, when he says "let Beauregard come here with a corps. We want everyone here we can get." I believe the idea being that this Corps operating in Northern Virginia would either keep the Union off it's toes or allow it to operate in the Union rear during the invasion. I will agree Lee should have pushed harder for both of these reinforcements.
To tie it up. Lee got approval for his invasion. Davis supported the invasion. Lee wanted a audacious approach where everything is sent to support the invasion that could be, Davis vacillates and never commits, and so Lee is left to do it alone. Even alone Lee is at as close to parity as he's ever been against the AoP, and so it's still a sound strategy. Sorry for the novel response LOL.