I'll try to answer your questions to the best of my abilities.

1. Yes, they can possess an animal as long as it lives (not dead). Do note that animals will fight back mentally as it felt its body being taken over, I imagine the smarter the animal, the fiercer the resistance.
2. Yes, they could have been a Kelvic in life.
3. I imagine yes, as ghosts have various appearances, mostly representing what they looked like in life with traits showing the circumstances of their death. A beheaded man for example, will show up as a ghost with a bloody stump of a neck instead of a head. Since the entirety of a ghost's body is soulmist, this means the soulmist can indeed have various colors.
4. Depends, what does the ghost know? If a Vantha with fluency in Vani and basic in Common died and became a ghost, he/she would also have those languages. A Dhani ghost would have Snake-tongue, Inarta ghost would have Nari, etc. It's the same as choosing languages for non-ghost PCs.