1. One of the Sage Advice columns specified that the Greater Magic xxx spells affect only one weapon (or group of ammo). This means that one casting affects only one sword, one claw, one unarmed fist, etc.
2. Greater Magic Weapon (and Magic Weapon) specifically do NOT affect natural weapons. Magic Fang spells only affect natural attacks.
3. Magic Fang is strictly druid/ranger - it's not on any of the core domain spell lists. It might be on a domain list from FRCS or a splat book - I haven't checked.
As far as the possible changes, lowering the bonus is greatly needed, but I'd also like to see them combine the spell effects into one spell, which can affect either a natural or manufactured weapon. There are too many situations (monks, non-human races for a pc, etc.) where magic fang is needed, but not available. A 1st-level Magic Weapon, and a 4th-level Greater Magic Weapon which affected either type would be an improvement, IMHO.