I attend military university and we were invited to the Gettysburg remembrance and then to Antietam. What is interesting about Gettysburg is that the South entered Gettysburg from the North and the North entered Gettysburg from the South.
If you have been to Antietam, you weren't that far from Gettysburg. Gettysburg is probably the best marked of the Civil War battlefields. I attended a class at Gettysburg College about 10 years ago on Lincoln and spent most of my free time going around the battlefield and didn't see all of it. The wife and I were there in June and a lot of the trees have been cut so it more resembles the battlefield as it was. At the time of the class, the latest body that had been found there that could be identified as a Civil War soldier was in 1997. My point is the battlefields, in many cases, were so vast that, as you say, many historians have the estimate at 700,000. It could be probable since many of the battles were fought around rivers, swamps, and largely deserted areas. Likely as not, many remain unaccounted for. I have a great, great uncle who died, and is buried, at Andersonville and if that many died in less than two years it seems the 620,000 could be low. If you can, go there as it gives an unmistakable visual reference of how many people actually died in the war.
As a Viet Nam Vet, I salute you: