"Untermensch" is a very complicated subject. For starters, Slavs are a linguistic category, not a racial one, and the Nazis were aware of this as well. Do you think they treated their Croatian, Slovak, and Ukrainian allies the same way they treated the Soviets? Of course not, and wait until you find out about Turkic, Arabic, and Indian sympathizers! Also, when the Nazis described the untermensch as a "leavening agent in all people", I think the concept was not so simple as "inferior races". The same language was used to refer to German communists, for example.

I think you laid it out well in the OP. Nazi opinions were generally divided and there was no official policy as to what was going to unfold after a German victory. You can point to harsh treatment of conquered eastern territory, I can point to the recall of ethnic Germans to the Reich territories. Why would you recall eastern Germans if you plan to send them back to colonize? Yeah, you'd be mistaken if you think Germans were not starving when their prisoners were starving, a large part of Lebensraum revolved around the fact that the Germans lacked the agricultural output to feed their population, which was a bitter memory from WW1 that reappeared in WW2. It's sad that people starved, though nowhere near as many as those that starved in the 1930s by the communists. I think that was intentional insofar as the German offensive was prioritized over the people in conquered territories, but not as an intentional plan to exterminate "untermensch". For every Himmler there was a Rosenberg, and for every Germanicist there was a "European Reich" supporter as you mentioned. I think Ostplan claims are atrocity propaganda to justify defeating the Germans after the war. You won't find any documentation outside of personal memoirs and speculation. And you can't pretend the Germans were a monolith who did not disagree on policies or future plans. In fact, if I recall, Hitler banned these speculative ideas in 1943 because they were wasting time and opportunities. Like you though, this isn't a big issue to me. You can find information on CODOH though, I'm pretty sure.