Do they eventually support themselves in the sand bed or what? How well do they spread and replicate?

Yes, they will eventually, assuming it is a deep, mature sand bed (some mud and fine sediments can help, though). They do have some energy reserves in the propagule that will last for a while--hopefully they settle by then. None of the setups I've seen show them reproducing at all. Most keep them in a bonsai form, so they don't really fully mature. Basically, you'll always have a single tree, unless you add more.

I also read they root well in porous rock. Assuming they were "planted" in a shallow tub amongst a bunch of LR I assume it would do well.

Yes, they can do that and can be found that way in the wild. I don't consider it as ideal a situation in an aquarium (mostly nutrient limitation in the water column in "lean" tanks), but is possible due to the tenacity and adaptability of these trees.

will root to anything they can hold onto. the rock doesn't need to be porous, the rock doesn't need to be rock