By my understanding, "the grave"/"death" is what happens to/ where your body ends up. Hades/Sheol is where your soul would go to "in the heart of the earth". I don't think that's so much a physical location, so much as a spiritual one that transcends the physical- I don't know how that's provable, but it makes sense to me. OT saints weren't able to go directly into the presence of God, because Jesus didn't do his thing yet- so they got to hang out in a more pleasant quarter of the underworld.
I'm not sure about Enoch, though... I wonder what precisely God did with Enoch. I have a hard time believing that "god took him" simply by killing him, but at the same time, I don't think he was so righteous that he was born into his spiritual body like some people say. It's a mystery to me for now.