Manure/compost seems to help keep the wood wet, in my experience anyway. However, getting enough material to cover the logs thoroughly is the biggest hurdle to
hugelkultur, and also one of the reasons I'm not so fond of it anymore. Wood chips are easier to cover, rot faster, attract more worms, and if you spread them out instead of making a solid layer then plant
roots can grow between them without trouble (logs not so much).
I would
not add sand to clay. Sand doesn't add anything of value to soil, and if you add it to the sort of clay you've got it makes a nice
concrete. Greensand is a whole different story, but I'd put that
under a sheet mulch rather than on top or mixing it in. Mixing and digging are worm jobs. On the other hand, a tall hugelbed covered with soil is so far from the underlying dirt that it's more or less like a giant pot, so things mixed under it may not do anything for plants for years. I've still got some small-medium branches that haven't rotted away after 2-3 years or so of being covered, and it's very warm and humid here.