My grandfather was the First Army IG in WWII, and one of his major tasks was to gather evidence for German war crimes. He brought home a stack of pictures from that. Among them was a whole series showing stacks of bodies, partially burned bodies in crematory ovens (they'd toss them in several at a time), and all manner of other ghastly things.
Having seen the pictures, I'm not impressed by your silly devil's advocate position.
My grandfather saw it with his own eyes, and I suspect he'd call you a fool for doubting it. The nazis were monsters, and they did what it's said they did.