1. Indeed, or the symbiont, depending on the type, feeds off the corpse. An inner symbiont would die, as the host cannot sustain it anymore, but an exterior would do as you said, ShadowFish.
2. Two characters could, perhaps, form a magical bond that allows them to do the stamnia-for-mana exchange, and so on. Anything that deals with mana, health, or stamnia, basically, but no \'special\' characteristics, as the characters probably don\'t have the special capabitlities to improve, say, the blood, in return for something else.
3. Agreed.