Even with fresh Kava you will still see a layer of separation if you let it sit. I've noticed this myself when using fresh frozen. Prepared kava is simply a suspended solid in water. Kavalactones are found in the resins which are more bound to the fibers of the root and definitely insoluble in water. I would dare to say if you let the solution sit for long enough you would see a complete separation between the water and the solids. To answer your question as to the percentages, I can't really give a solid number, but I'd say most of the kavalactones are located in the heavier particles at the bottom. Not all, but most.
The solids are the particles I seek after dehydration. It's where instant comes from.