The sentence that contains an incorrect shift in verb mood is the second one: "Wait for half an hour after dinner, and then you should have dessert."
This sentence contains two verb moods. "Wait for half an hour after dinner" is in imperative mood. That is to say, it is a command. The second part of the sentence is not written as if it were a command. It is written as a suggestion. "[Y]ou should have dessert" is in its conditional mood. You can realize that because of the use of the auxiliary "should".
