I have the just one explanation you consider Ukrainian as the East Slavonic and Russian as the South Slavonic - the Ukrainian is POLONIZED dialect of the Russian. And it become more and more polonized. Just add one more thousand polish words and it will be a new East Slavonic language.
Everything above is based on your theory. Now lets work at your mistakes:
1. East Slavonic Group - Russian, Belorussian, Ukrainian, Rusin, Old Russian.
2. West Slavonic Group - Polish, Kashub, Czech, Slovak.
3. South Slavonic Group - Bulgarian, Makedoniad, Serbo-Croatian, old Slavonic, Church Slavonic.