The aluminum expands, not the hole in the aluminum. Heating the steel insert would cause both the spark plug threaded insert to expand and the aluminum head. The threaded hole in the head would shrink in response to heating. Your suggestion may work, but the hole will not expand.
Did you contact Saturn yet? It should be their responsibility. Even though the dealer is supposed to represent Saturn, that doesn’t appear to be the case here. If the first person you talk to can’t help you, ask to speak to a supervisor.