Wenever an invisible of hidden thief make a melee attack, he make a backstab.

I'm pretty sure that the thief also has to be behind the target. In BG 1 that wasn't necessary, but since TOSC they fixed that. You also have to have the proper weapon equipped. I know daggers work, small swords might as well.

You don't need to be behind.

As for what weapon work, it depend. Dagger and short sword all work. I know the staff of the magi does, so probably all staffs do. Club also do. The list is long, the best way to find out is to try them.