girning over trifles (pg. 35) -- To complain in a whining voice. To contort one's face; grimace.
arrived in a caléche (pg. 129) -- A light low-wheeled carriage with a removable folding hood.
leave the door on the snib (pg. 164) -- A snib is a manually operated catch for the internal locking of Yale type locks.
practicing a phrase on the Pleyel (pg. 317) -- Not an instrument, as I first thought, a piece of music by Ignace Pleyel.