Magic Arrows: however you want.
I treat them like spellbooks or scrolls. A one-time use item, or something that can be used at least once per day depending on the Talents you take.
If a player wanted to create a "Fey Hunter" sort of character I would have them take "Scholar of the Unseen University" and start them out with 3 enchantments they could place on the arrows in their quiver. If they wanted more utility out of them the "Manifold Cerebrum" would also be a good choice.
I might say that when they chose to use a magic arrow it was like they created a psychic bolt, rather than using a mundane arrow that round. Stuff like that.
Missing Spell Durations: Whoops. I always default to [L] or [HL] rounds/Turns depending on what makes the most sense. The duration of the encounter also works perfectly well.
Banish: This is specifically an undead thing for people who like that aspect of D&D clerics. Though you don't need to be a cleric to take the Talent.