Was Sickles justified in disobeying orders when he moved down from Cemetery Ridge to position his corps along Emmitsburg Road on the second day?
What would have happened if he had remained on Cemetery Ridge as ordered, would there have been less casualties, would the line have held the Rebel attack?
No one can know for sure. My opinion has usually been that of viewing the results of an action. We cannot know what might have happened for moves that did not, in fact, happen.
Lee's plan seems to have been to attack up Emmitsburg Pike, and roll up Hancock's Line onto the Union Cemetery Hill, Culp's Hill Line.
It is well to remember confederate corps were oversized in comparison of the AoP and III Corps was badly depleted from its actions at Chancellorsville . . Very few Union cops could take on a confederate corps without help., especially if surprised by a full blooded assault on its flank.
Sickles' Corps had to spread itself dangerously thin to just reach the base of LRT(not occupy it), i.e., he had no mass at any point of his line to receive a serious attack.
Lee, in his reports
I believe mentioned in his reports that the high ground at the Peach Orchard gave his artillery a clear line of fire on Cemetery Hill , i.e., it dominated Cemetery Hill and Sickles' assigned position on Hancock's flank.
Lee's attack was to be on the flank of the AoP and the veteran Hood was disconcerted when he reached his jump-off position for his division , supposedly to the South of the Union Line and found Union troops to his front. What disconcerted him, was not that there were Union troops there, but that they were facing his line of attack, in Line of Battle.
Instead of a full blooded, coordinated assault on the Union MLR, with full artillery support and open ground for the assault to gain momentum. The attack was stalled at its very beginning and turned into a slow grinding battle of attrition., that took the entire afternoon to overcome, prolonged, whether by chance or plan, is unclear to me, by Meade feeding in just enough reinforcements to lengthen the battle without endangering his Cemetery Hill defenses
P.S. it seems that Lee's failure on Day and 2 was cause for blame. That it might have been done better, is an assumption, based upon other assumptions. If anything, the confederate failures on all 3 days of the battle rests on Lee and his faulty system of command; if Lee had been able to command Longstreet, the attack would have began before III Corps was even in position in its original position, even before Sickles' made his movement forward, in the first place.