Prussians would exchange Posen for Saxony without hestitation, problem is no one except for Prussia and Russia was interested in such solution (especially Austrians opposed it). It would require war between Russo-Prussian coalition and Austria, France UK, which almost happened BTW.
And November Uprising was not winnable, it could last maybe one more year and Prussian involvement could be needed to crush it, but it was doomed. And even by some miracle Congress Poland get full independence, what next? Without access to the sea and Russian market it would suffocate.