1. In behalf of: for someone's benefit or in someone's interest.
There was a charity dinner in behalf of the poor children of Chicago.
On behalf of: To act or to speak as the representative of someone. Somebody speaking on your behalf is expressing your opinion or thoughts in your absence.
Dr. Reese accepted the award on behalf of Dr. Sullivan.
2. In the meantime: The interval of time between one occurence and another, during the intervining time.
I am going to the store to buy groceries in the meantime you can clean your room.
3. Whom: Acts as the object of a sentence.
Elena is the girl whom I met in NYC.
Who: Acts as the subject of a sentence.
Elena is the name of the girl who works in Mr. Garcia's office.