I got the idea that the treaty was signed in 1863 because Craigo mentioned in his post for the Great War peace treaties that the US recognized the Confederacy that year. The issue of returning slaves was honestly not up for debate. Craigo's post on the Fugitive Slave Treaty made that pretty plain.
Doesn't the very fact that a Fugitive Slave Treaty had to be enacted in 1867 suggest that the issue wasn't addressed as part of the original peace treaty?
Anyway, I just don't see how it is mathematically possible to get 2/3 of the senate willing to support returning slaves in 1863/1864. Lets look at the math.
The 37th Congress ended with 30 Republicans, 12 Democrats and 7 Unionists. Those numbers are pretty much locked in from before the TL-191 POD, so they should be more or less accurate.
Now in the 1862 senate elections, incumbent Republican seats were up in Connecticut, Maine, Massachusetts, Michigan, New York, Ohio, Pennsylvania, Rhode Island, Vermont, and Wisconsin.
Massachusetts, Vermont, Rhode Island, and Maine are all extremely Republican in this period, so there's no chance the Democrats win any of those seats. As such we are looking at a maximum Democrat gain of six seats. (And even that's being pretty generous as its rather unlikely the Democrats can take Wisconsin.) So even under the worst case scenario, the Republicans will still hold 24 senate seats after the 1862 elections to 26 senators for the Democrats and Unionists. (Kentucky presumably being allowed to keep its senate representation until the treaty is ratified.)
So even if we assume that every Democrat and Unionist will vote to ratify the treaty, it is still going to require 8 Republican senators to also vote for it, and 8 Republican senators aren't going to support a treaty that requires surrendering escaped slaves back to the Confederates.
The situation is very different by 1867. By that year there have been further senate elections in 1864 and 1866, so the Democrats would presumably be much closer to a 2/3 majority than they were 1863/1864. (IIRC Craigo had the senate breakdown in that year as 30 Democrats to 20 Republicans.) And there's a Democratic president in office in 1867 who can use patronage and pork to try and get Republican senators to support such a treaty. (Something that President Lincoln would obviously not do as he would despise any treaty condition that required returning freed slaves.) Thus it is far more plausible to get a Fugitive Slave Treaty enacted in 1867 than it is to have such a provision included in the original peace treaty.
The Union already repelled Confederate invasions of Missouri long before the point of divergence. Only the western tip of Maryland was occupied.
Very true, but I would expect the Confederate fire eaters to still make demands on both states. (Especially Missouri since it was actually included in the Confederate Congress.) The U.S. obviously ultimately prevailed as to both states since neither joined the CSA, but the negotiations over them would probably have been lengthy and might have led to the U.S. making concessions in other areas. (Maybe the U.S. agreed to give up having Kentucky's status being decided by a plebiscite in exchange for the Confederates giving up their claim on Missouri.)
And if nothing else the failure to obtain Missouri and Maryland in the negotiations will probably be a sore point amongs the Fire Eaters and a major rallying point for the anti-Davis faction in the CSA. (Indeed it was likely a major theme of the Beauregard campaign in 1867 with Beauregard insisting he would have handled the negotiations much better and obtained Missouri and Maryland.)
