I've long thought that the key to a pro-freedman Reconstruction that wouldn't have been rolled back by the "Redeemers" would have been both arms and land... But if slave revolts in slave-majority areas resulted in their having both an effective armed militia and de facto possession of the land, and if the slave owners they rebelled against happened to be held to be guilty of treason against the USA, then the legalities might wind up conforming with the realities on the ground.
Yes, land is the key. It's possible to turn yeomen into serfs, but it's a lot harder than doing the same thing to tenants.
That's what was missing from the Sea Islands Experiment in OTL - the freedmen never had their land tenure recognized, and after the war, the plantation owners got it back. The freedmen who didn't leave ended up as tenants.
In this timeline, the manner of their liberation will be different, and political expediency will give them control over where the land goes. The Sea Islanders will come out of the war as yeoman smallholders which, especially when combined with other arrangements, will put them in a position to keep some political control. There will be one or two other places in a similar situation.
(And for that matter, the freedmen might not take only de facto possession - if they set up entities capable of granting de jure possession, the Union might force the postwar state governments to ratify these entities' acts.)
If reconstructionists could also build up an alliance of the poorer whites with these freed people, then the Redeemers might not be able to force their way back into power, in at least some Southern states, particularly ones with lots of African population, like Louisiana or Mississippi. Or South Carolina! There'd be black Senators, and a fair number of black Representatives in Congress.
The freedmen wouldn't even need that alliance in South Carolina, where they'd be close to 60 percent of the population; all they'd need is to be able to forestall the kind of political terrorism that occurred in 1876. Militias made up of county smallholders might do the trick. In LA and MS, they'd need alliances, which would depend on whether the poor whites put greater emphasis on the "poor" or the "white."
I would say no matter what happened in the other Southern states, but unfortunately Congress and I believe the Senate can refuse to recognize individual legislators and refuse to seat them, so if a sufficient number of other Southern states followed a Jim Crow path and found sufficient Northern allies, they could exclude entire states.
This was never done in OTL, even after the Redeemers took over the South; the last black Republican Congressman from North Carolina left office in 1901, and none of the representatives elected during the 1880s and 1890s were excluded. I'm not sure if the Redeemers even seriously tried. They might make the attempt if there were more black Congressmen, but I can't see the Republicans up north (or even many of the Democrats) going along with it. As you say, the African-Americans will be considered worthy supporters of the Union, which won't count for enough to prevent the Redeemers from coming to power in some states, but would probably preclude things like keeping South Carolina out of the Senate.
Now the big difference between this situation and OTL would be if significant tracts of land were self-liberated by slaves repudiating their bondage. OTL of course southern African-Americans did a whole lot to support the Union cause and would have done more if Lincoln and other Northern leaders weren't afraid to countenance their rebellion. Whereas I'm not sure even the Malê influence would be enough to consolidate really sweeping slave victories [...]
So far only the coastal islands of Carolina are being foreshadowed as likely bastions of slave self-liberation, and that's just about OTL too. So unfortunately I don't see a lot of hope that the situation will be a lot better for Southern ex-slaves than OTL, in most places. It would be quite something though if the Redeemers tried to take back SC, and failed.
There won't be really sweeping slave victories, but the Sea Islands won't be the only place that self-liberates, and even a temporarily successful slave revolt could create domino effects elsewhere. And as noted above, the Sea Islanders - and possibly some others - will end up with land as well as the ratification of certain political arrangements made during the war. Whether this will be enough to make Reconstruction stick at the state level - and I agree that South Carolina is the state where this is most likely to happen - remains to be seen.
Or more pessimistically, we could have some of black majority areas being treated like Indian Reservations (with all of the attendant problems that would come with that) after the ACW.
Less like Indian reservations than like quilombos or Maroon colonies (or the colonias of southern Texas) - mostly left alone, but also left out of whatever development plans the white-dominated government might dream up. Overcoming this will be a major part of this timeline's civil-rights movement. As noted above, though, South Carolina may be at least a partial exception.
The Bavarian army was integrated in the Imperial command structure and the King was commander-in-chief only during peacetime, and AFAIK, the Bavarian diplomatic corps consisted solely of an envoy to the Holy See (which wouldn't be an important issue for Hanover). I doubt that Hanover could achieve much more, so if we assume that the TTL German Empire is similar in structure to OTL's, real leeway for an independent Hanoverian foreign policy or own colonies seem unlikely.
Does it make a difference that Hanover will be in the North German Confederation, not the German Empire? The Empire's constitution wasn't much different from the NDB's, and the NDB was well on the way to being a state with a unified parliament and courts, but it was at least in theory an alliance rather than an empire, and the king of Prussia was at least in theory first among equals rather than overlord. So if the NDB continues to exist after a drawn Franco-Prussian war, would the more privileged member states have some autonomy over foreign policy? In other words, would there be Prussian and Hanoverian colonies, but no "German" colonies as yet? Once the NDB becomes an empire, of course, the member states' colonies would be subsumed as German colonial possessions.
If that's not plausible, then would the member states of the NDB be autonomous enough to charter their own state companies? If, as we discussed a while back, the Congo basin became a Portuguese colony with the proviso that other powers have the right to bid for concessions, could Hanover or even one of the free Hanseatic cities be among the concessionaires?
Failing either of those, I think I'll stick with the "Prussian win in 1866, draw in 1870-72, rematch in the 1890s" scenario.
I am really interested to see how you play Reconstruction now! It would be great to see a steady and continuous national or state at least Black political class develop in the South from the Civil War on, even if very restricted.
Most likely there will be a steady and continuous class at the county level, and a less-interrupted presence in some states (although not others) - a few more "Black Seconds" scattered around the South, maybe.
One pitfall here is that with the freedmen getting kicked out the most natural group to draw a Black political class from is being liquidated. Of course there are still natural leaders among the slaves, but the lack of educated freedmen will be a handicap (of course some will come in from the North after the war but not having lived there all their lives will be a disadvantage).
Not all the freedmen will get kicked out. The laws will vary from state to state, and as in OTL, some freedmen will be able to secure individual exemptions.
On one end of the scale, there will be states like Maryland and Louisiana where the free black population was large enough and well-established enough that expelling them would be logistically difficult and economically disastrous. On the other are states like Mississippi and Alabama where the freedmen are so few (and in Alabama's case, mostly concentrated in one city) that they aren't as much of a threat, and where the state government will focus more on restricting the freedmen and preventing new manumissions than expelling them all. The places in between - Virginia, North Carolina and to a lesser extent South Carolina - will crack down hardest, and even there, enforcement won't be 100 percent.
Also, some of the slaves will be educated - for instance, a certain gentleman from Charleston will play an even larger part in this timeline's Reconstruction than he did in OTL. And some of the freedmen who had to leave the Southern states before the war will come back during or after. There will be a base to draw from.