just IMO, but that's a no to both, for me. Of course, everybody has their own favourite way of doing it, so it's just a question of finding what you prefer.
AFAIK there is never anything really to gain from soaking kava, won't hurt it any, but I believe that simply in the process of blending you will rehydrate the kava which happens pretty much instantly, so it doesn't require soaking. Soaking just means you have to wait longer before having a shell.
I've blended with milk, cream, half and half and coconut milk and I don't honestly believe the fat made a difference. I still use coconut milk diluted with water for my krunk doses but only because the coconut milk improves the flavour. It's pretty optional. With the Manoa prep I just use water and have no problem drinking that down these days, as long as I have a few sips of ginger beer to chase it or some pineapple.