You really think that they read Mein Kampf

Doubt it, but possible. Most likely they were told an "abridged version", so to speak.

felt that Lebensraum was a good idea

Sure. How do you think Russia got so big?

felt elated at being described as subhuman?

Just like the modern nazis in Ukraine, they likely thought it wasn't about them. Blamed it on the Jews and godless commies (who were either Jews or manipulated by Jews). Here you'll say it's just supposition, but here's the thing, I live in Russia, and I had the displeasure of knowing a number of Neonazis personally. Classmates and such, you know how it is with groups you can't leave. That was basically their shtick. And those historical collaborators that weren't actual, fervent nazis, they thought they were liberating ol Vaterland from the ebil judeobolsheviks. Just look at the amount of "former" tsarist officers and aristocrats that had gone on to collaborate with the nazis.