I agree. Gettysburg clearly demonstrated how much Lee had counted on Jackson. Jackson would have taken Culp's Hill on the 1st and the Confederates would have been able to rain enfilade fire down on the strength of the union line.
As it was, on the 3rd the Union had Culp's and two Round Tops and did the same to Picket's division and the other forces in that charge so Lee had no chance of success.
I've been there and looked out across that field from the Union line and it's a long way to march under fire. I'm amazed that any of them got back to their line alive.
I think the battle was "Confederate winnable" on the 2nd day, with a couple of changes to Lee's battle plan. The plan lost time, marching troops south to rush Little Round Top at the same time an attack by Ewell was taking place on Culps Hill. Needless to say, the time/motion of this plan doesn't work. The idea was to weaken the center by attacking both ends, but as there was no way to coordinate the attacks, the center strengthened first one end, and then the other. Lee should have kept the troops on the north end and mounted an attack on Culp's Hill with double the numbers.
Another road to success for Lee, on the second day and on that battlefield, would have simply been to be more creative. In the past, Lee and his lieutenants had done wonders with creativity. Just two artillery pieces up on Big Round Top, which Lee held at the end of the first day, would have made the Union position untenable. Lee's men had worked miracles with handspikes and axes before -- at Gettysburg, they didn't even try.
But Gettysburg was a turning point in another way, too. You mentioned the loss of Jackson, and it by itself was huge, but the CSA Army was beginning to suffer from a loss of good leadership across the board. Good CW commanders led from the front -- and people leading from the front were likely to become dead -- and given the South's population of warfare-eligible men, there was little likelihood they'd be replaced. There was no time to train-up new "middle managers" and no source of them if there'd been time. Victory for the South could only happen at that point with an influx of leadership and materiel from another source, as happened during the Revolution when the French helped Washington, and Europe provided leaders such as Lafayette and Von Steuben.
Lee could have won on that battlefield, but his army was decaying and the Union Army was improving. Even if he'd won, it would have been non-military matters, such as recognition of the CSA by France or England, that would have changed the outcome of the war.