To be honest, Aura doesn't really drain. Think of it like an invisible set of armor. As long as a tiny piece of armor remains, it can still protect. In this case, as long as there's a tiny bit of Aura left, Linx and Kiir can still use their abilities.
However, much like armor, after it's pierced, the regular flesh is exposed. In Linx and Kiir's case, once you pierce a part of their Aura, that part will be exposed, and any further attack in that area will actually damage their flesh (if they're not wearing armor. Linx is however, soo). Keep in mind that their bodies are still at least street level in durability without Aura. That means wherever they don't have Aura, that part will have street level durability, making them glass cannons.
But, it'll be incredibly hard to pierce because Aura acts basically like an added durability, and will only be damaged with something on the same tier, let alone cut through.
As for Willpower Defense, I know it's not immunity, but at the same time, Linx and Kiir's resistance to said abilities is 90%, so...