A what if:
If Longstreet had Picketts men up and could have extended the Confederate right on day 2, would that have been enough to overlap and break the federal left?
McLaws thought so. He contended that the assault on day 2 should have included Pickett. But Pickett couldn't reasonably have been expected to get there - his division had just arrived at Marsh Creek the afternoon of the 2nd.
"If Pickett's division had been with mine following it, I believe that Round Top could have been captured from my side and we could have established ourselves there. But if Longstreet was waiting for Pickett, he was not allowed to wait long enough, because General Lee did not think the enemy's left was occupied so strongly as it was, even at that late hour, and was not made aware of the great natural strength of the enemy's position. If General Longstreet had taken the responsibility to report that the positions in his front were naturally so strong and were so strongly occupied that his force could not accomplish the important results that were expected, and insisted on a delay until his whole force was concentrated and a more thorough examination made, I do not think the battle would have been fought at all, but that General Lee would have maneuvered to force an attack upon himself." [Lafayette McLaws, "The Battle of Gettysburg," a paper presented before the Georgia Historical Society, January 7, 1878.]
Of course, that would have only been potentially successful if Pickett was nearby enough to join on Day 2 and he wasn't. The longer the delay, the more reinforcements arrived and the stronger the Union left became. Since Pickett wasn't there yet, then to await Pickett, as McLaws suggests, would have delayed the assault until the Union Sixth corps had arrived and been there, in force. McLaws and Hood did some good execution against two corps, but who knows how it might have turned out with Longstreet's corps up against three entire Union corps? Or would it have been four? Either way, not good.
The charge as it was made on July 3rd simply makes no sense without Anderson or McLaws following up the attack.
Regarding day 3 and McLaws being able to actively participate or assist in Pickett's Charge, you really need to read what McLaws says about it in that excerpt I suggested the other day.
https://civilwartalk.com/threads/view-of-picketts-charge-maj-gen-lafayette-mclaws-on-july-3.159910/
McLaws was 1/2 mile from Pickett's right, with a strong reinforced enemy in his front. I'm no tactician and even I can understand what that means. Had McLaws abandoned his position (holding the front of the now reinforced Union left) to join Pickett, then this would have surely been exploited by the Union forces in his front. The Union left in his front would have come forward and enveloped the Confederate right. His and Hood's divisions would have been rolled up. With no force remaining on their left to prevent it, the Union center would have followed up after a failed assault on the center. Game over for the entire ANV.