Not sure how Douglas's position is stronger without the debates as opposed to with them.
Douglas would never issue the Freeport Doctrine that alienated Southrons if Lincoln didn't press him on Slavery. Indeed, the whole series of debates is regarding slavery. If Douglas is able to present himself as a "middle ground" in 1860, he may well be seen as the last chance for Union. It helps that Lincoln would probably not be the GOP nominee in 1860 as well.
But it may be possible, especially as the course of the war will obviously have changed from OTL if McClellan has won the 1864 election. If the Union public have voted for a Democratic Party in which Copperheads and peace advocates are in the majority (as demonstrated by the platform the party adopted at its Chicago Convention and its choice of George Pendleton for VP), then the people are clearly voting for peace, having decided that bringing the Confederacy back into the Union fold is not worth the price being paid in blood and treasure.
Moreover, if McClellan and the Democrats do adopt a policy of "Reunion With Slavery" in an effort to bring the South back in, what will be the reaction of the Republicans? I would imagine that they would cease to support the war, as they would see it as not worth winning if slavery is going to be permitted to survive.
The stated goal of the Democrats in 1864 was to stop the fighting and start peace talks with the aim of bringing the South back into the Union through negotiation. Since McClellan has won the election in this scenario, we can assume that the war went better for the Confederacy than it did IOTL and they are in a much stronger position in early 1865. If the fighting stops and negotiations begin, I cannot see any reasonable way for the fighting to resume when the Confederates reject terms that don't acknowledge their independence (which is what they would do). The Republicans probably wouldn't support resuming the fighting and the Peace Democrats wouldn't want to in any event. The Confederacy would be independent in all but name.
The reverse is more likely. Lincoln would support reunion with slavery if he had to choose between abolition of slavery and rebuilding the Union. Privately, he felt deeply conflicted about what to do if the Confederacy made that offer. If he lost the election, he'd probably loyally support the war to rebuild the Union, and he'd be able to swing much of the moderates with him.
Of course, its easier to get to 1865 with a viable confederacy if Lincoln never makes it to the White House. I'm not sure who is president in 1861, but unless they've burnt all their bridges demanding emancipation right off the bat they'd probably bend towards Union first--after all, if the Union wins, Slavery's time is still numbered. It's not like the Union will return thousands of 'contrabands' to their owners in Louisiana or Tennessee. There will be no shot of expanding Slavery to Kansas, or any more territories.
If the South agrees to re-entry to the Union and keeping slavery for itself, it's conceded the question it faced in 1850 and beyond--how does it intend to keep the slave bloc as powerful as the rest of the country? There will be no more balance in the Senate, in all likelihood a long period before there is a Southern President, and definitely a lot of distrust throughout the rest of the nation. It holds slavery; perhaps Virginia is reunited, and Confederate Debts and Money are probably not scraps of paper.
Now, maybe in 1864-5 that's a reasonable conclusion. Louisiana is gone; Arkansas and Texas have been severed from the Confederacy; Tennessee is partially occupied. Clearly, the South can't win if McC decides to invest four years to prevail. Perhaps it takes until 1866, no longer, for the South to accept McC's terms--and they may well become less generous as well.