Could Lee have been decisively defeated.
Yes. Although it was very rare in the ACW for a field battle to be truly decisive. At Winchester in 1862, Stonewall Jackson attacked with over twice as many troops, and completely routed the army of politician-general Nathaniel Banks. The Yankee survivors rallied on the other side of the Potomac, 50 km away. But Banks lost less than 1/3 of his troops.
At Antietam, the Union clearly had the power to break the Confederate lines and drive the rebels off the field. But they didn't have the rebels
surrounded, and they didn't have an effective cavalry force to ride down all the fugitives. Most of the rebels would have escaped - either via the one ford over the Potomac to the west, or west and then south along the Potomac. Hill's division, which marched up from Harpers Ferry just in time to save the Confederate right, would have showed up, done some damage to the Union left, and then withdrawn; most of the rebel stragglers would have rallied on that force.
If so does the CSA fight on.
Yes. Davis would not give up. The Union would have a great advantage in the Eastern Theater, and might be be able to capture Richmond by the end of 1862, but the rest of the Confederacy would be largely intact, i.e. nearly all of NC, SC, GA, FL, AL, and MS, half of TN, most of AR and LA, all of TX.
There would be a considerable morale effect, but OTL the CSA fought on into 1865 under hopeless conditions; even a crushing defeat at Sharpsburg would not make them all give up immediately.
On paper if they re-joined the Union by Jan 1 1863 their establishment figures get to keep holding people as property.
As would all other slaveowners, not just the elite. But I doubt that any of them believed such a move would get them any real protection for slavery.