The spell Swarm Skin is unclear about the damage taken while under the spell. If a player takes 30 pts of damage under swarm skin, does she have that 30 pts of damage when she returns to her body? A Crab Swarm has 38 hit points, if a witch memorized the spell twice and the damage taken by the swarm does not transfer it would be the equivalent of 76 extra hit points. The only time she could actually be hurt is when she returned to her body (unless someone targeted the bones).
considering magic jar is a lower level spell and when using swarm skin you can't take any non swarm actions (like casting spells) extra hit points dont really seem that big a deal
RAW supports the swarms are taking the damage, not the caster's bodily hit point total.
Considering it is a level 6 Spell that can be used to turn in to low level swams. I think it is as problematic as saying if you dont go adventuering you wont be killed in a dungeon. It is a Nice thematic Spell but i dont see it being used for other things than stealth stuff. Or to harass pesants.
It does not seem like the swarms loss of hit points affect the casters hit points at all.
Even if someone destroys her bones, the witch in the OP's example, would still be "alive" in swarm form and only die when the swarms are destroyed.