Why a war between Austria and Russia at that time should have been a more serious disaster for Europe than the partitions of Poland?
This war would be certain devastation for Poland in first turn, because it would carried on principally in Poland, for, as you rightly mentioned, it was buffer state. However, it is a question which state Poland would sided in this conflict. Partition in this situation was the lesser evil.
A natural bufor zone between them vanished and as soon as the three partitioning "black eagles" had stopped being preocupied with the threat of the Polish state being restored, they gripped themselves by their throats
The fact is that Poland always was buffer state against
Russia. Simple fact that next conflict between Russia and Germany took place only more than one hundred years later, clearly indicates that all partitions and eventual dissolution had been right things to do.
Quite contrary, war of 1812 perfectly shows what independent and "buffer" Poland may be for Russia.