1.The ethnonym "Ukrainian" was already widespread on the lands of the former Cossack Hetmanate by late-18th and early 19th centuries, as evidenced by the writings of Kotlarevśkyj and Ševčenko.
2. Your idea of "South Ruthenia" is interesting, however, South Ukraine wouldn't be Russian in such a case. The settlement of these lands was not primarily russians, it was a collective effort of Ruthenians, Russians, Greeks, Jews and Germans. While the cities were indeed mixed ethnically, the countryside was by far majority Ruthenian.
3. You claim to be a universal nationalist, yet you also deny the existence of Belarusians and Ukrainians, which is stupid since a Nation is primarily defined by 1.Language 2.Land 3.Consciousness. Both Ukrainians and Belarusians not only have languages which are distinct from Russian and Polish, but also live on a fairly consistently defined area and have considered themselves separate since the 19th century along with all the other National Identities forming in that period.
Also add, I guess.