But would the Russians accept Krakow getting Galicia instead of themselves?
Good point. I guess it depends whether this TL occurs simpy because the Habsburg elite has the leap of insight PoD that ImperialVienna masterfully devised, or we get a second PoD that temporarily paralyzes Russia that I mentioned upthread (assuming it is not a rebellion in Poland, which would make them active and paranoid about such a rump). If the latter, the Habsburg can set up their Krakow-Galicia rump Poland, and after Russia recovers, it shall be forced to live with the fact. If the former, it may well be that Russia claims Krakow and Galicia as its slice of flesh for going along with the Habsburg plan. Then we would get the Habsburg bloc shunk down to Germany, Hungary, and italy, and Poland would have to wait for the *Crimean War or *WWI to get liberated.
OTOH, there is a critical difference in this unification scenario: everything is happening peacefully, on the leadership of the proper authorities getting a world-changing epiphany. If the Habsburg decide to divide their heritage, and the German and Italian states to accept federal union with them, it is not the other powers' business to interfere. Russia can certainly claim Krakow and Galicia as a balance of power compensation, but stricly speaking, it has no legal claim to it. France, the only power that has a real stake about stopping a mega-Habsburg bloc on its borders, is paralyzed by revolution up to early 1849. Britain is neutral or sympathetic to a liberal Germany, Italy, and Hungary (and definitely sympathetic if they turn anti-Russian). If the Tsar chooses to press the issue to the point of war, he would find isolated against a liberal Triple Alliance, with Poland up in arms again, and Britain at best an hostile neutral. In such a fight, I see Russia getting an epic smackdown, even more so if Britain decides this the right moment to declaw the bear.
I like your idea of partitioning Austria and getting an independent rump-Poland. If Germany then accepts partitioning of the Posen province - or at least a referendum - you could get a Polish rump state consisting of Galicia and eastern Posen, a potentially loyal ally to Germany and sworn enemy of Russia.
Hmm, given that a sizable German minority existed in Posen, I'm rather skeptical sure that German liberal-nationalists would accept putting them under Polish rule. They would surely accept giving Posen a remarkable degree of autonomy, and perhaps a division of the province, but not giving the whole of it away. However, I notice you talk of a partition. Yes, I think that this could be done and could be a valid compromise to settle future German-Polish relations on a good track.
Of course, such a a rump Krakow-Galicia-eastern Posen Poland would make the Poles deliriously happy (they now have their "Piedmont" or "Prussia", with a set of powerful backers), but the Russians mad. Germany and Hungary are unloading their share of Poles to an independent rump Poland that, given its odd size, only really makes sense as the irredentist nucleus for the Russian lion's share. I can really see the Tsar declaring war on this.
But as I said above, this quickly turns into the Habsburg liberal crusade against Russia, Poland explodes, Britain is buying popcorn and sending money to the liberals, if not thinking to lend an hand. France is mired in revolution and would have political trouble fighting to defend the Tsar anyway (not to mention that Britain could easily intervene against a Napoleonic-Tsarist alliance). Germany and Sweden are currently at odds about the Schleswig-Holstein issue, but quite possibly Sweden could become interested about making a compromise, perhaps with Denmark keeping northern Schleswig, and joining efforts to liberate Finland and the Baltic...
Even if a war doesn't happen now, the relationship between the Habsburg bloc and Russia shall reach polar temperatures, with Poland a nationalist hotbed worse than preunitary Italy, which really spells trouble for Russia whenever Britain or its proxy Japan decides to get aggressive. France shall be caught in a geopolitical dilemma, between conceding hegemony of the continent to the Habsburg bloc without a fight, or a kamikaze attack on it, with backward Russia as its only ally, which promises a smackdown even worse than 1870.