Yes, if you miss your smite attack, it is used for the day.
No, even if the text is lightly different, this ability works exactly like the Paladin's Smite Evil => if the celestial creature miss, the smite is used for the day.
Almost that but Magic Items creation also requires skill checks (appropriate craft or spellcraft). The DC ca be increased to ignore a pre-requisite for the item creation.
A failure on the check can lead to the creation of a cursed item.
1. No change : you must use adequate PrCs
2. Almost no change (PrC BAB and ST rule has been modified IIRC). Using fractionnal BAB & ST should be a good idea I think.
3. Spells have been modified : SoD deal damage, SoS have a save every round.
4. No change : spellcasters are still SAD if that's...