All that was needed to prevent Sickles' movement was for Longstreet to organize and begin his attack anytime from 1100 hrs(when Lee originally planned for the attack) until the middle of the afternoon (when Sickles' finally decided on his move) .
I don't think there was any way Longstreet was going to get his men in position for an 11 am attack, under the best of circumstances. Just to give you a comparison, one July 2 we were giving a program at Devil's Den and had about 300 attendees assembled in the parking lot at Big Round Top. We wanted to get them down the hill to Devil's Den and took them down the Boy Scout Trail that leads down that way. Single or double file was the best we could do. It took more than half an hour to move 300 people less than half a mile. Picture moving thousands of troops, lots of artillery, getting them placed, from the area of Chambersburg Pike and Marsh Creek to the area near the Peach Orchard, some four miles.
According to Coddington, Longstreet ordered his corps (about 17 miles west of Gettysburg) to march to Gburg at about 10 am on July 1, but they did not move until 4 pm, reportedly because Johnson's Division (Ewell) and the Second Corps wagon train had the road blocked. McLaws reached the Marsh Creek area at about midnight, Hood reached about 1 am. Artillery was delayed by wagons on the road and left for Gburg between midnight and 1, arriving at the Gburg area around 8 or 9 am. Law's brigade and Pickett's division left early in the am of July 2. Longstreet's troops stopped at Marsh Creek because that's where the water was and they were in desperate need of it. Getting all those men and equipment to the battlefield four miles further east takes time.
In the meantime, as dawn broke Lee had people checking out the terrain. Among these was Johnston and his party who went down near the Peach Orchard to check out LRT. Union troops were not there yet but were nearby but apparently Johnston could not see them because of the woods. Around 9 am Johnston was finished and went looking for Lee. Around this time, Pendleton told Lee that Sickles's troops had arrived and were in the lower sections of Cemetery Ridge. Lee still thinks no one is on LRT itself.
About 10 am Longstreet met with Alexander to arrange for artillery and troop placements, and they could see there were Union signalmen on LRT. Lee and Longstreet did not want his troop movements seen by these signalmen. Lee gave Longstreet permission to wait for Law's Brigade to arrive (Pickett was nowhere nearby yet) and when it did at about noon, Johnston began to lead them south toward the Union left flank.
Johnston led them, staying on ground that was not visible to LRT, to the Black Horse Tavern on Fairfield Road and then onto a road that would take them toward Willoughby Run as he gets closer to Marsh Creek. Suddenly (and I know it's sudden, I was just there on Friday), this low ground rises and is visible from LRT. That's when the countermarch back to find another way to move south unobserved began. They have to go back to Black Horse Tavern, move across fields and take down fences, to find a way, and in the meantime they have to sort themselves out because they are getting all tangled up.
The odd thing is that when the Union signalmen spotted the Confederate troops, they had already started their countermarch and were headed north toward Chambersburg Pike. Coddington doesn't say this and I honestly don't know if Sickles knew that the report was that the Confederates were moving north, but in any event, they turned back south and found a way to get down to what is now the Millerstown Road area, and by the time they did, Sickles had moved.
Meantime, of course, among the Confederates everybody is fighting with everybody else. Longstreet is mad at McLaws and Lee, Lee is mad at Longstreet, Ewell is unhappy because Lee wants his men moved south and then doesn't want his men moved south....... and the troops are hot and tired and out of water. The miracle is that anything got started at all on July 2.