Hmm. So with Expensive Gift, as the second part is an independent sentence ("Gain one Soulstone.") would that part go off, or is it something like "If you did make a target with Use Soulstone lose a Soulstone, then you can gain one Soulstone"?
And related, with Magician's Duel, if the target did not Use Soulstone would you still be able to do the "If the target is killed by this spell gain one Soulstone"?
I guess it is the same question posed twice - does the second part rely on the first part?