What's the best way to defrost ice-cream/sorbet quickly?
Don't.  Ice cream is hard.  It melts slowly.  Instead, focus on scooping.  

Get the largest spoon you have, or ideally, an ice cream scoop.  Fill up a cup with boiling water, or as hot as your faucet will get it.  Dip spoon/scoop in the water.  Scoop.  Dip.  Scoop.  Shake off excess water as you go.

Like a hot spoon through ice cream.