There was another possibility to Corban's attack that Hermione hadn't considered in all of the excitement and then the confusion she felt with Draco. Maybe Corban's attack wasn't Rodolphus at all. As the Head of the Department of Magical Law Enforcement, Corban had a lot of power in the Ministry. With all of the chaos and disorder in the Ministry of Magic thanks to the death of the Dark Lord, it was entirely possible, maybe even probable, that there were people within his own department that had something to gain with him no longer in the picture. Hadn't Ron even said that he had a number of allies within the auror department? They couldn't afford to solely focus on one potential suspect or they ran the risk of becoming short-sighted and ineffective.