1) You can add extra properties to any wearable object that doesn't already have three or more properties. (The following objects apply: Weapons, Shields, Armor, Hats, Cloaks, Boots, Belts, Bracer, Gauntlet, Rings, and finally Amulet.)
2) Yes, while presuming the condition from the first question are met.
3) Yes, while presuming the condition from the first question are met. Although the item weight reduction consumes one enchantment slot. For example, you can only add two enchantments to a Mithril weapon due to the pre-existing weight reduction from material.
4) You can buff up weapons that already have enchantment bonuses. The higher always apply. An Eldritch Knight that has his weapons enchanted with only "on hit properties" can use his spells to get the usual weapon enchantments.
5) The extra properties are added one at a time. And you can improve an existing property as long as the condition from the first question is not met.
Edit: Regarding Darksteel, you can upgrade the 1 point electric damage to a 1d4 or 1d6 of electric damage.
Stupid Crafting Queston
