Put 4mil plastic on the joists, covering the entire underside of the house. Then put 4 mil of plastic over the dirt and gravel. Make sure the crawl space is properly vented. As for the residual CS odor, replace the furnace filter, purchase a couple of bags of crushed lava rock and hang them near the return ducts. Most hardware stores have them. If not, save a couple of the mesh bags onions come in, get a bag of lava rock for grills, crush them as best you can, put in mesh bag and hang them. What's great about lava rock is, when they absorb odors and become saturated, you hang them in the sun for a few hours and they're as good as new (last several weeks). Hardware stores also sell an air freshner packet that attaches to your furnace filter. There are also activated charcoal filters if you want to spend the money. The lava rock works great.

Hot water.

You could also use a matching hardwood transition strip. I've never liked the brass look for floor transition strips, myself, and they also tend to dent and don't age well. Although there may be some that age better than others. -Roger

I would contact the slab yard or fabricator you worked with and ask for their recommendation for your specific marble. I'm sure marbles vary in which are more porous, etc. Also, since you have it in your shower, the treatment for areas exposed to water daily may vary(I'm almost positive that would need to be sealed more often than say the floors)