Found it!
The process of emancipation started May 23,1861.
This was 41 days after the CSA chose to start war at Sumter. It continued pretty consistently up till the last slave was freed.
I fail to see how a continuous process starting less than 6 weeks after the war began can be construed as an ex post facto justification tacked on “towards the end of the war.”
1861
May General Butler refuses to return three slaves being used to build CSA fortifications to their owner. Concept of “contraband of war” invented.
August Confiscation Act of 1861 declares that any property, including slaves, used by CSA could be confiscated by military action.
September “Contrabands” employed by US Army and Navy paid wages, in addition to rations
November Nathaniel Gordon convicted and sentenced to death in NYC for slave trading (classified as piracy)
1862
February Nathaniel Gordon executed
March Washington, DC slaves freed by Congress, with partial compensation to owners
Return of escaped slaves to their owners by army officers prohibited by Congress. Even slaves escaped from Unionist owners.
April Congress offers compensation to any state that emancipates
May Lincoln publicly appeals to the border states to free their slaves
Slavery prohibited in all territories
July Lincoln appeals again to the border states
Militia Act of 1862 frees slaves who enlist in US military, their mothers, wives and children. Initially applies only to slaves from disloyal states or owners.
Second Confiscation Act provides for enticing slaves to leave owners in slave states, thereby becoming free. Authorizes President to issue Emancipation Proclamation as exercise of war powers.
September Preliminary Emancipation Proclamation
November Attorney General Bates issues ruling that free blacks are US citizens. Opens door to slaves of loyal states and owners freeing themselves, their mothers, wives and children by enlisting in US military.
1863
January Final Emancipation Proclamation issued 20,000 to 50,000 slaves in Union-controlled territory not specifically excluded by the language of the EP freed immediately
July WV slaves freed by state action
1864
January 13th Amendment introduced
March AR slaves freed by (puppet government) state action
April 13th Amendment passes Senate
June Congress repeals Fugitive Slave Law
September LA slaves freed by (puppet government) state action
November MD slaves freed by state action
1865
January MO slaves freed by state action
13th Amendment passes House
February TN slaves freed by (puppet government) state action
April Lee surrenders. All slaves freed in former CSA territory not excluded from the Emancipation Proclamation.
December 13th Amendment ratified
Slaves in KY (~50,000) and DE (~200) freed
You are going to call a Union General's refusal to send back workers to the Confederacy as equivalent to initiating emancipation?
That is silly. I dare say no General would send anything useful back to the enemy. As a matter of fact, it would appear the General initially regarded them as "property", which kind of blows a big hole in your argument there.
August Confiscation Act of 1861 declares that any property, including slaves, used by CSA could be confiscated by military action.
And this just reinforces the point that they were regarded as confiscated property, and not free men. The Union wasn't even buying their own press releases at this point.
September “Contrabands” employed by US Army and Navy paid wages, in addition to rations
And here they realized just how silly they look and so they put a fig leaf on it after the fact.
The rest of this stuff is just attempts at rationalizing various events as being equivalent to a post hoc casus belli. Anything which happened after July 21 does not count as the cause of the Invasion. A lot of this stuff is just tactics to weaken the South militarily while strengthening the North Militarily.
None of this explains why they bypassed Maryland on their way to invade Virginia. What was the difference between Slave owning Maryland and Slave owning Virginia?
One of them was independent, the other was not. It is that independence which was intolerable.