I think you're severely over-estimating Junker power...in a victory early in the game. Which I believe is what the man proposed. Your points are valid for a late CP win, I will not argue them(atleast no the meat of it).
I'm an admitted Prussophile, this is well known, and it colours my visions of the future of Poland under German guidence. It could be horrible, but I like to think that people tend to be reasonable, and won't exploint them to the point of poverty. Not in the long run, anyway. Excesses will certainly occur, but I genuinely think this is one of Poland's best chances at being a secondary power in Europe, especially if we can get a more liberal German parliament/chancellor who may cede parts of the Posen province. Not the whole thing. Posen itself is too German as a city, But some border era? Not out of the question. As for the Austrians...their voice will be small at best. Germany was the dominant power in the CPs before Austria started having problems, Austria post-war will be propped up entirely by German arms. They aren't getting a significant amount of Ukraine, and I don't see why they wouldn't put a Hohenzollern on the Romanian throne.What I think people are forgetting is that while German will recieve the lion's share of Mittleeuropa, there's so much wealth to exploit the natives are bound to get a boon from it. Not much of one, but you could very well see a sort of Autobahn system in MittelEuropa, or at the least a very advanced network of trains.
Hardships will occur in the beggining, but I honestly think that this would be best for Poland. Congress Poland plus some select parts of Galicia are all it deserves, they're the only areas with a Polish majority. It won't be as grand as the second republic, but it also won't be run by that madman, tyrant, and general scum of the earth Pillsudski. The Intermarium would be no more than Poland lite Nazi Germany. Trying to Polonifie the the conquered peoples and getting rid of the rabble. He was no better a person than Mussolini, and in Pilsduski's more unhinged moments, Hitler-lite.
Slow evolution is always preferable to Przestrzeń Zyciowa
Again there will be explotation under German rule, but it won't be some proto-nazi nightmare. It won't be perfect for a while, but with the advances in educated spurned on by the likes of Simiens, a Polish intelligentsia will grow leaps and bounds.
I'm not trying to paint this as rainbows and roses coming out the ass of Kaiser Bill, but I think the picture of the Junk class is sorely biased. They weren't all evil. They wanted concessions, but nothing -too- outrageous in most cases.
The OP in question proposes a German victory after a five year front in the West... that really doesn't seem like a quick victory IMO, but to each his own.
The Kingdom of Poland (that is, Congress Poland) would definitely fare better as part of Mitteleuropa compared to being part of Russia. At least the Russification policies will cease and the Sejm will actually be able to meet again once the war is over, so Poland will certainly have a semi-democratic government (I say semi merely because any constitution will surely have influences from the Constitution of the Empire -- although considering the war against France ends after five years, the German Constitution will probably get some amendments to curb the excessive powers of the executive branch).
I never said that Poland would suffer as part of Mitteleuropa, merely that Poland in it's borders minus the lands held by Germany and Austria and quite small... and lacks some of the key zones that could be economically prosperous. Aside from Poland, there's also the other portions in the East included in Brest Litovsk-- I'm assuming that means the Baltics, Lithuania, Belarus, and Ukraine. I don't believe I ever said the Junkers were evil either, but they certainly played a great role in Wilhelmine Germany whether you want to admit it or not. They definitely had political clout, given their great opposition to Caprivi's free trade policies against their own protectionist policies. Especially if Germany has a friendly Ukraine on it's side, a practical breadbasket on it's own, the German agrarian class is going to want some guarantees that they're not going to be undercut by cheaper foodstuffs coming out the east.
I also really cannot see Germany ceding border areas in Posen to Poland, even if the Empire manages to democratize to an extent. Even that small piece of land still leaves Poland quite small, and not to mention still leaves Poles within the German Empire in areas like Danzig. If they cede portions of Posen province, what is to stop Poland from demanding other border areas where Poles are in the majority?
And yes, Austria is a junior member of the Central Powers Alliance, that much is obvious. The OP doesn't specify what happens to the Habsburg empire -- does it collapse? Does it survive and shaky ground and undergo change into a federation -- or perhaps more likely, a confederation where Emperor Karl remains head of state and there remains a common army and navy, but the respective kingdoms, Bohemia, Hungary, Croatia, Galicia-Lodomeria have more extensive autonomy? If anything, the inclusion of Galicia-Lodomeria into Poland would be way more useful than some border territories in Posen. Austria would even probably be willing to let it go if Poland elects Karl Stefan as their King, as he essentially raised his family to identify with Poland, spoke Polish, and even had connections to the Raditzwills IIRC. A Poland with that territory would have a chance to be a good junior partner to the German Empire. But not in the rump form that's just the Congress Kingdom and some pieces of Posen and a few bits in the east.
I'm hardly painting a picture of the Junker class... I'm not sure where you're getting that from. You said they wanted concessions... the exact same thing I said. They're going to want assurances that their products won't be undercut by cheap eastern foodstuffs. That hardly sounds evil or outrageous to me.