We still don't properly understand what factors determine resistance to aetheric corruption, and that seems to be the point of the current story arc. We'll have to wait and see how that pans out.

I'm not sure if it's possible to deliberately corrupt yourself without external assistance. Most magical disciplines seem to be built around maintaining some sort of aetheric balance. If you'll recall, Red Mages still have to balance Light and Dark aspected aether in order to properly cast spells. Likewise, Black Mages and Thaumaturges make use of Umbral Ice to replenish their aether reserves despite using Astral offensive magic.

Conjurers are primarily concerned with cleansing the land of aetheric corruption by dissipating corrupted elementals, and the quest line acknowledges that misuse of these powers can throw the elements out of balance. I haven't seen much with regards to Doman Geomancy outside of the Astrologian quests, but they seem to be built around a similar principle. Interestingly, this is not dissimilar to the approach we take to restore aetheric balance with Eden, or the approach Azem takes to prevent the volcanic eruption in 'Tales from the Shadows: Ere Our Curtain Falls.'

There are plenty of other magic systems out there that are likely aetherically based and subject to the same rules, Ninja Mudras included. But a word of caution - we still don't know how Dynamis fits into any of this, and whether there are job disciplines capable of achieving 'magical' effects without actually using aether.

I don't think that perfect aetheric balance is essential or achievable. Historically, your Guardian from the Twelve conferred different elemental affinities, but I'm not sure if that is still canon now that the system has been removed.