1) I say it functions as a "normal shield" in this respect. To me, that means it can be used against one attack (we also allow it against all attacks from one opponent, and in rare cases against several attack from the same position, like both a wolf and the orc riding the wolf). No extra activity for using the SHIELD BONUS (+25) against a missile attack, but then you can't use it against another attack that round. For an ordinary shield, the shield can only be used against attacks on your "shield side", that should probably be the case for the spell as well.
IN ADDITION, the spell, just like any other shield, allows you to missile parry. To me, that's something ELSE than applying your shield against an attack. For example, in melee, we allow players to use the shield against one opponent, and attack/parry with the weapon against another opponent. The opponent you use the shield against, could very well be using missile. MISSILE PARRY is when you PARRY an incoming missile attack, by reducing your OB. It is MINIMUM a 50% activity. I say if you only use 50%, you can parry with 50% of your OB, if you want to use 100% of your OB, it takes 100% activity.
2) I think I'd only allow magical effects to destroy a shield spell.