Yes, the amp rating is a measurement of the highest amp load it should carry. So a 15 amp appliance will work on a 20 amp socket but you wouldn't want to use a 20 amp appliance on a 15 amp socket.
Short answer, Yes. However, pulling that much from a circuit may cause problems if there are other appliances plugged into the circuit. It may cause annoying circuit breaker tripping if the 10 amp...
Yes, if properly wired a 20 amp circuit is a great circuit for an 18 amp appliance. Properly wired means you've used 12 gauge wire or larger and the circuit is protected by a 20 amp breaker or fuse...