Why was it established as a federal rather than a unitary state when the party was thoroughly centralistic and unitary?

I suspect that suggests part of the answer. Because the party was very carefully maintained as a centralized unitary party and because Lenin and Stalin saw it as the principal power center in the Soviet Union, the question of federalism versus centralized government did not mean all that much. The party was going to assure that whatever government there was at Kharkiv or Kiev would not step out of line, would do what the party in Moscow wanted it to do.

Consequently some concessions could be made. On the other hand, the federal system as it was established did not make sufficient concessions to the Ukrainians who were part of the political structure in Ukraine, and there was a good deal of grumbling and discontent on their part. Once again the answer that Stalin came up with was that additional guarantees would be made in the direction of Ukrainianization to accommodate the Ukrainian nationalists who felt that the federal structure was drawing them under Russian authority.

Q: What ... more.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.