Why does Mark's father employ Cordelia to find out why his son had allegedly killed himself if he was the one who killed his own son?

Sir Ronald Callendar hires Cordelia Gray to remove any doubts about his own involvement and to find out who changed his son's clothes after he was murdered.
After the people Sir Ronald hired to kill Mark finished the job, Mark was not only dead but painted with makeup and dressed in lingerie. That was what the man expected; it was how he wanted Mark to be found. However, when Mark's body was found, he wasn't in makeup or women's lingerie.
When he realizes someone else involved themselves in the murder, he hires Cordelia to investigate. In this way, he both looks less suspicious and has the opportunity to find out what happened. Ultimately, hiring Cordelia was his downfall. She finds out the truth—that Sir Ronald paid for people to kill Mark and ruin his reputation to make sure that Sir Ronald didn't lose an inheritance. After Mark's body was found, the person who found it removed the makeup and lingerie so that Mark wouldn't be humiliated in death.