Ooh, this topic is one I have a lot of feelings about.
I think there are a few smaller things you could do:
a) Three confederate soldiers eventually served on the Supreme Court so enact a law preventing anyone who fought against the Union from holding office;
The 14th Amendment (Sec 3) did this OTL for anyone who had previously sworn to uphold the Constitution. But the Congress lifted this restriction in 1872, ie while still heavily Republican and before the economy went pearshape. The political will for anything like that just didn't last.
b) Strike down the restrictive "black codes" passed by the South in the Supreme Court basically as soon as they're passed;
On what grounds? The 14th Amendment was still in the future, and in 1865 Congress had yet to pass any laws to enforce the 13th. In any case the Codes were all done away with under Radical Reconstruction even OTL. Why would doing so a year or two earlier make any difference?
c) Designate the Confederate flag as a symbol of an enemy nation;
Which makes a difference how? Did the KKK and related organisations normally fly a flag when they went out about their business?
d) Instead of paying slave-owners for their loss of profit, provide stipends to slaves as reparations for their years in servitude
Slave owners were never paid for loss of profit. The 14th Amendment (Sec 4) expressly forbade that. And giving money to Freedmen would have involved raising taxes, which would be high enough anyway to pay for the war - quite ASB.
.And then the bigger one, ensure Samuel J. Tilden wins over Rutherford B. Hayes
Huh! Tilden was a Democrat and would have pulled the troops from the South just as quick as OTL.
My professor mentioned that there was a plan to take land away from former plantation/slave-owners and give it to the newly freed in the form of "forty acres and a mule" and then open up the rest to be purchased by anyone. OTL that did not happen, of course. It'll probably be hard to pass.
There was no "plan". During the war, the Union Army found itself coping with huge numbers of slaves who had escaped during the turmoil of war or whose masters had fled, and as an emergency measure some of these had been settled on abandoned plantations. Iirc, Congress later passed a law allowing Freedmen to be granted land with "such title as the United States can convey", a form of words reflecting their knowledge that permanent confiscations were almost certainly unconstitutional. Thaddeus Stevens and a handful of others played with the idea, but it was never government policy under any Administration, nor even remotely likely to be.