1. No, because Dorothy’s statement was not an offer, but rather a statement of her intention to contract in the future. An offer requires the present intention to enter into a contract; here, Dorothy did not intend to create in Wicked Witch the immediate power of acceptance. Therefore, no contract results.
3. Yes, probably. If a reasonable person in Oppenheimer’s position would have no reason to know of the value of the plutonium (or any other reason for thinking that Einstein was joking), then under the objective theory of contracts, Einstein’s offer would create an immediate power of acceptance in Oppenheimer. The fact that Einstein actually was joking is irrelevant; it’s the appearance of a valid offer that counts. If, however, Oppenheimer knew that Einstein had a very dry sense of humor, and realized at the time that Einstein was joking, there’d be no valid offer and thus no contract. (An “offer” which the offeree knows is made in jest is not a valid offer, regardless of what any other “reasonable person” might think.)