Sorry, but as far as the Baltic territories are involved, this is a complete fantasy because it means dismantling one of the victorious Great Powers: if Alexander agrees to a doing something of the kind, he may easily end up with one of the deadly diseases from which his grandfather and father died.
Status of Finland had been defined by the agreement between Russia and Sweden and not a subject of discussion at Vienna.
AFAIK, Alexander was the only one who insisted on having at least some Polish state and the idea of it as a buffer state (between whom and whom) belongs to post WWI period and “the Red Scare”: neither Prussia nor Austria wanted any Polish statehood. Why would the victors voluntarily give away parts of their territories to award a nation which was actively involved on the wrong side of a conflict?
This was the issue for me as well. There doesn't seem to be a reason that Russia would support this arrangement (other than Congress Poland), but I put it out there to explore if there could be a feasible set of Vienna scenarios where Poland gains greater autonomy.