Comey never argued that Hillary didn’t break the law, but did not recommend prosecution because there was no intent. So all they need is an email or other evidence that proves intent, and her goose is cooked.
Comey's legal interpretation was flat out wrong.
The espionage act specifically says that gross negligence alone, with or without intent, is a violation of the statute.
But in any case it is a moot point. One can never really know what is in another person's mind, so the the question of intent invariably requires specific tests for its application, which all prosecutors use. One of them is action of apparent "guilty knowledge." Clinton repeatedly refused requests to provide evidence, and by her own admission deliberately destroyed evidence. That alone is proof of intent in most courtrooms.
Comey's legal interpretation was a facile rationalization to do what his masters told him to do.