Effects of failed foreign intervention in U.S. civil war?

Hard to say. Lincoln's reconstruction plan was moderate, and with the 10% plan and a protection of non-slave property, almost weaker than Johnson. But at the same time, these rebells just dragged America into a war with Great Britain (or france). That's gonna piss people off, even moderates.
Or perhaps he could give swathes of Canada and other newly acquired possessions out in a sort of homestead act specifically for african americans.

Indeed. At the same time, that anger will be partially mollified in the end, as without the traitorous actions of the rebels, the Union would never have achieved their borders in the north with taking the majority of Canada for themselves. In victory, the Union would be more magnanimous than in defeat.

At the same time, they'd definitely be dealt with more stridently, especially as this will inflame the Unionist sentiment in the southern states as they invite foreign interlopers on board. Some Confederates would be wary about becoming a British pawn even as they relish the aid. And, as it becomes apparent that the Union is being victorious, the border states and similar will start jumping ship one by one. And, as I mentioned before, those that do switch will be treated better than those that weren't.

All that being said, if the Union wants to use the Emancipation as a cudgel against the British, they might certainly begin opening up homesteads for former slaves more than OTL. It may not be completely effective, but it would also sap the South of much of the power base required to act up once again (the slaves inevitably becoming sharecroppers).
 
In this timeline, probably either Maximilian would survive the Mexican adventure and have a son (though he was 35 in 1867 and would have been 85 in 1916 when Franz Josef died), or butterflies keep Rudolph from committing suicide, so Franz Ferdinand or any other children of Karl Ludwig would not be the target of assassins.
 
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