The first round alone was proof that Jiraichan wasn't an impostor. Since there were 2 dead crew members (Siruko-san and Goto-san) and the round was short, it means that both impostors killed 1 crew member each. During that time, Jiraichan was with Hanae-san and never left his side. His alibi was very tight. Kensho-san's alibi on the other hand, contains a lot of holes. His timing of pressing the emergency button and Hakotaro-san being dead on the other side of the room was already suspicious all on its own. (If that wasn't enough, Shoichi-san's laughter was a huge giveaway that he's not innocent.)