So all one of the Sidhe needs to do in order to toast a mortal is to lob said mortal something valuable first, then incinerate them while they're still wondering what the hell is happening? Nice.
For extra fun, use a fire-proof trinket, that way it's reusable.
Uh, no. They need to give the human something legitimately valuable, as valuable as the price they charge. Like, say, saving their True Love's life (the fact that you indirectly arranged for her to be in danger is irrelevant), that you can get some serious power over them with. Leah helped save Harry from He Who Walks Behind, which is why his debt is so big, just for example.
And most Fae can harm mortals even without this sort of thing, only the Queens
need it for that...but all Fae need it to
own them.
I'm pretty sure the mortal has to accept the gift, too. So they'd have to catch the bauble, but even a reflexive catch would count...