I've implemented higher levels of cure/inflict spells, lazy Wizards.
Nope, I based it off destruction. It leaves the soul but the body generally has a bad day.
They die . They either walk it off, get staggered from blood loss, or hit the floor.
Yes. Only magical items, copying the 'soul' of their magical properties into the ghost soul.
A caster can allow another to use their body, such as for making skill checks when locked in a cell. They can have the party rogue take over their body in order to open a lock, for instance.
Switching between different types of casters. "Surprise! I'm the Evoker in the Necromancer's body!"