LOL. Illegimate President? You sound like a Gore-bot.
Not that I want to keep raining on your myths, but here's the facts.
10% is a "slim margin" in your world? The Electoral count was even more lopsided.
Some other "facts" you probably don't care to know. McClellan did not support an "end to the war" other than through victory. He was a "War" Democrat, not a "Peace" Democrat. He did oppose the freeing the slaves however which is why he won in the slave states of Deleware and Kentucky.
As to the results being different if the South had been able to vote, assuming all of their 80 votes would have gone the War Democrat McClellan, the Electoral Count would have been 212 for Lincoln and 102 for McClellen.
I should point out that Lincoln didn't take a single southern state in 1861 and still won with almost 60% of the electoral vote. Had the southern states voted as a block for McClellan in 1864 then Lincoln still would have won 212 to 109.
Jefferson Davis, on the other hand, was appointed in February 1861 and ran unopposed the following November. How legitimate is that?