Harlath, good catch. To be strictly accurate, on page 99 it says that a Gandalf symbol or a 10 cause a piercing blow, and the adversary must make the save vs. THAT or suffer a wound, so it is not automatic. Success die 6's can be spent to add to the feat die to reach ten.
But like I mentioned before, the attack has to hit first. If you roll a 10 and miss, it's not a piercing blow at all.
And yes, you can use tengwars to add to the feat die if the weapon has the Pierce special damage ability. However, once again, the attack has to be successful
before you can spend tengwars to boost the feat die.
If a successful attack roll produces one or more Success icons, they can be used to trigger one or more
special results, based on the war gear of the attacker
So yeah, an eye is always a hit, but if you have a good enough Parry, enemies can definitely miss you even if they roll a 10.