Great question! After doing some digging around and thinking, based on these pages,
EV-enhancing items are items which enhance the EVs gained when a Pokémon is defeated. All of them halve the Speed of the Pokémon holding them in battle while they are being held.
However, in battles that do not give any experience (such as in the Battle Tower or if the Pokémon is level 100), Pokémon will not gain any effort points.
It seems that if a Pokemon is defeated after the item is knocked off, then it will not gain the extra EVs. While there is no concrete proof of this, it can be justified in this way: The item causes both reduction of speed and increase in EV simultaneously. So if one effect (speed drop) is nullified due to knock off, it is safe (but not necessarily 100% accurate) to assume the other effect is dropped as well.
Again, working on assumption, EVs are added after the Pokemon faints, i.e,. if opponent is defeated, all participating Pokemon get EVs and Exp, upon defeat. If the party is wiped out (war flashbacks of a Level 70 Quiver Dance Volcarona destroying a young trainer's hope in Black), then none of the Pokemon get the Exp or the EVs. If an item is not present at time of defeat (which triggers addition of EVs in the first place, ideally speaking) then it can't effect the number of EVs.