The correct answers are The security guard was asleep at his desk. and The shiny diamond disappeared exactly at midnight.

Every English clause needs exactly one subject. If you already have a noun acting as the subject (like "The museum director" or "My partner"), adding a pronoun (like "she" or "he") right after it creates a "double subject" error.

Some languages drop pronouns entirely because the verb endings do all the heavy lifting. In English, we must use a subject, but we only need to state it once!