Would the Soviets have done the same to Germany had the US not been a rival superpower?
While I don't disagree with a lot of your points when applied to some hypothetical different TL, they quite clearly aren't consistent with the text of TL-191. Americans don't see Confederates as subhuman in the text, at least not to the extent you make out, and there is never any indication that the US is dishonest with its plans for the former CSA in IAtD.
I get that you don't like TL-191 and think its ASB. I respect that view. But I don't think your criticisms really work in this context. Your proposed TL is interesting, and I'd recommend you do something with it (just changing a few names of characters and making it less obviously TL-191 derived would make it an interesting TL in its own right), but if you're going to make points about something that aims to follow TL-191 canon I think you need to work within its assumptions, otherwise you aren't really being fair to the text presented. While I respect that you don't think TL-191 canon is something worth following up on, its the basis on which I'm making this TL and the mod, and its something people are still interested in as a scenario. That's never going to change, and poking holes in the foundation won't impact that.
I 100% believe they would've ethnically cleansed even more Germans, and Poles for that matter, yes. IOTL the Russians flat-out annexed Koenigsburg, and big chunks of Poland, which entailed massive ethnic cleansing, and moved Poland's borders west in a way that involved more forced resettlement (which the US didn't object to enough to actually stop because nobody wanted to defend Germans after the leader of Germany ordered the murders of 11 million people and caused the deaths of countless more in a war of aggression). The USSR was perfectly happy to force all sorts of minorities inside its official borders to move, too, and that was at the height of the Cold War. Ethnic cleansing is a favorite tactic of imperialists and revanchists since time immemorial, and Stalin was no exception.
As I've established, I think the text is crap and any attempt to move forward has to deal with that in some way. Whether by trying to forge a more realistic aftermath going forwards or just rewriting the text outright is up to the individual.
The problem is that TL-191's assumptions are completely internally inconsistent and serve only to give Turtledove more books to write. The entire thing is just flimsy justifications for Turtledove to fight WW1 and WW2 again in theaters that make no sense for those sorts of wars, and he flat-out changes which country is paralleling which halfway through (remember McSweeney and the Redeemers? The US was obviously supposed to be Germany and the Confederacy some mix of Russia and France, with the Confederacy under a reformer Featherston becoming less racist and awful just in time for a radicalized fascist US to invade).
By TL-191's assumptions:
--The Confederate leaders are A-OK with industrializing, democracy, and freeing their slaves, the former of which is done by getting nebulous loans from the British and presumably offscreen import substitution.
--The Confederacy is able to beat a Prussian-style US army to a standstill and lose only some peripheral territories in *ww1, despite having no local allies with any significant military ability and its major allies being halfway across the world engaged in a major continental war and apparently not suffering any food problems because Turtledove doesn't care that half of the average Briton's calories in 1910 came from the US Great Plains.
--Exact parallels for Adolf Hitler, Heinrich Himmler, and presumably most of the Nazi big names except Rommel who's American all show up and take over the Confederacy in a beat for beat retelling of the Nazis' rise and the Great Patriotic War.
--The Confederacy is despite vastly inferior numbers somehow more industrialized than the US, or at least can muster a massive tank army and run a genocide on a scale significantly larger in absolute and proportional terms than the Holocaust at the same time despite having no natural industrial region like Germany and the US do.
--The US is even more racist than OTL despite having no reasons to be that way and every reason to be anti-racist for nationalistic and propagandistic purposes.
--At the same time the US is engaged in a nonsensical Iraq War expy in Utah where stereotypical terrorists are messing around, and apparently they are not willing to do to the Mormons what they did to the Native Americans despite having every reason to do so and no reason not to (the Mormons were hated almost as much as Native Americans, them being white means effectively nothing).
--Corollary to the above, apparently the majority of Americans are anti-interventionist liberals, just like they are in Turtledove's
Man with the Iron Heart, which is just as silly and just as eye-rolling in its apologia and nonsensicality.
None of this makes any sense whatsoever and it's internally inconsistent to boot.
What I'm saying is that the US absolutely can and will treat the CSA like it did the Lakota and Neshnabe and whatever other people you care to name. Its only serious rival cannot possibly hope to contend against or outproduce even a divided US occupying Canada and the former CSA, and it is allied with the world's only other real superpower. It has no major overseas colonial empire and a large population with a prodigious, even incomparable industrial core. There is no power to support the Confederate version of the Provos (diplomatically or otherwise), and no reason for anyone to sympathize with such an organization since they would immediately be tarred as Featherston sympathizers even if they didn't overtly claim Featherston's Great Man legacy.
The Confederacy is dead, and it would take a miracle for it to be reborn in any form.