Good start. I agree with Lee creating a third corps if it looked like Jackson would be out for the campaign, and this scenario has a last-minute appearence. But I suspect this would be a Jackson more like the Seven Days, or worse yet, a fatigued and anemic commander like Lee at North Anna or Hooker after his concussion at Chancellorsville.
I suspect Jackson would have blinders on going into the battle, just like everyone else. There's no Stuart, and no Turnet Ashby. His scouts would be operating under others, along a broad line in hostile territory. If he did see the long line of Union forces, I think he would be less likely to push aggressively on July 1, rather organizing to hit an isolated part in force. Tighter command on either side would have resulted most likely in avoidance of the battle, maybe until late on the first.
That's my contrarian take. Also, I don't think the Union command would have feared Jackson. Meade held his own in his prior engagements with the II Corps, as did Reynolds. Howard's men would have held as well as they did, the OTL attack was hardly a sudden ebvelopment.