Turns out I was slightly wrong about how their invulnerabilities work.
I was foolishly trying to use the Amtgard Rulebook to figure out what natual invulnerable worked when I should have been looking in the Dor Un Avathar.
Natural Invulnerable Armour is treated just like normal sectional natural armour except each point is an invulnerability. Is healed by the energy heal and is mendable (not healable).
Guess I can take more arrows to the jimmys next time before dying