I think it was Bruce that did a test cook a while back that found that the temps eventually even out and become the same. Others have said that they never do. It's usually not a big problem either way and if you want, you can flip your meats during the cook.
Buy 'play ground' sand sold and home improvement stores and I think it has all been sterialized. Alton Brown does his in the oven. You need high heat to actually sterialize the sane=d so the WSM probably isn't the best place to do it.
I use water, sand, an empty pan, a pan with aluminum foil in it, and somethimes no pan at all. Experiment and decide what you like and want to use. Treat the pan and it's contents just like another tool in you BBQ arsenal.
"Of all the imaginary friends I've had, I don't think there was one that I didn't end up having to kill."
in seach of Umami