Its generally accepted that Lee's incursion into Maryland was a play to garner the the support of the Southern loyalists there and gain a lot of new recruits. Had he won at Antietam he felt he would've been seen as a liberator freeing MD from Yankee oppression and that was supposed to result in waves of goodwill from the locals. As it was he dropped his battle plans and ended up leaving the battlefield non victorious. I believe the war was totally winnable right up to the third day at Gettysburg. The North was becoming war weary.
I totally disagree with your contention that the CSA could have won the American Civil War up until the the July 3, 1863.
IMO the USA was superior to the CSA in terms of political leadership, population, and logistic infrastructure. The only way that the CSA could have won was if one or more European powers had chosen to support them with the war materials that their feeble infrastructure couldn't product in either volume or quality. Lee's two invasion of the Union were failures that bleed the Army of Northern Virginia of men and materials without producing any tactical or strategic advantage for the CSA. If McClellan and Mead had been half as aggressive as Grant, either of them could have destroyed Lee's Army during its retreat back to Virginia. I doubt the the USA was any more or any less war weary than the CSA at that point in time, probably less so since few people in the USA were facing any shortages and their land wasn't being fought over. The great majority of American Civil War fighting took place on CSA soil, not USA soil, even if all of it was American soil.
