The correct answers are Who ate the last slice of chocolate cake? and Who did the suspect call before fleeing?

When the question word (like "Who" or "What") is the subject of the sentence, we do not use an auxiliary verb like do, does, or did. We just use the normal verb (e.g., "Who ate...").

When the question word is the object of the sentence, we must use an auxiliary verb before the subject (e.g., "Who did the suspect call...").