[Endoperez]

The limit is actually "if the item before this one is the same item, don't give bonus". This is because having more than two miscellanous slots is a relatively new feature, and few one-handed weapons have any effects worth taking a Nataraja for.

As long as the ability in question stacks, yes. However, Quickness (from the spell) is the spell's effect, and it's only ever applied once. Some of the XXX Brand-swords offer elemental resistances IIRC; they could be stacked if there was a shield between the Brands.