[Alneyan]

Missile protection actually makes you immune to Seeking Arrows; at least, my mage was not hit once after a dozen or so castings.

Luck helps as well, but will not always be enough to negate the arrow; it seems to make the "for some reason, XXX managed to escape his deadly fate" occurrence much more likely. Etherealness does not seem to help however, and does not seem to make the "lucky throw" more likely.