Card Control

If I take control of an enemy's monster can he use it's effect when it is destroyed if the effect is this "When this card is sent from the field to the Graveyard: You can Special Summon 1 Level 2 or lower "_____" monster from your Deck."

Yes, because the effect activates in HIS or HER graveyard.

Yes.

Cards with an Effect worded like "When this card is sent from the field to the Graveyard: You can...", are activated by the Player in whos Graveyard the card is in.

Example: You take control of your opponent's "Dandylion". You use it for the Xyz Summon of "Numer 17: Leviathan Dragon". You detach "Dandylion" to activate its Number 17's Effect.
Dandylion's Effect will activate for your opponent.

SIDENOTE: If a card would be worded like this "When this card is destroyed and sent from your side of the field to the Graveyard, you can...",very few cards are actually worded like that, its Effect would not activate in such a case.

Example: I take control of my opponent's "Dimensional Alchemist", which is worded as I've stated above. I summon another Monster and my opponent activates "Torrential Tribute". Dimensional Alchemist's Effect will not activate since it was sent from 1 player's side of the field to the Graveyard of another player.

Hope this helps!