The idea that the Empire was strictly xenophobic never felt right to me. I mean it's a big galaxy with a history that includes a thousand generations of Republic rule across a million worlds. You'd think things would be pretty cosmopolitan. If nothing else humans would be collectively outnumbered a trillion to one, which seems to make an all human military an untenable idea.
Not really at all. If the civilian population isn't organized on any level militarily and the humans have a futuristic military with Star Destroyers and Death Stars, it's not going to be much of a fight. The technological advantage demolishes the argument that skewed alien to human ratio would prevent human dominance.
You are basically trying to fit a square peg into a round hole. Putting a conclusion before evidence. The original idea of a racist empire was derived first from the evidence of the military being mostly composed of humans.The officers are a bit a a problem, but I'm willing to overlook it for the same real world practical reasons I overlook that the vast majority of rebel soldiers being human.