Is a person who attempted to kill someone, then discovered the person was already dead before the act, subject to Islamic legal punishment? And what are the evidences for that?

The worldly punishment for murder is referred to the Islamic judiciary. As for the punishment in the hereafter, the warning includes whoever intended to kill, even if he did not commit it; due to the saying of the Prophet, peace and blessings be upon him: "If two Muslims confront each other with their swords, then both the killer and the killed are in the Hellfire!" This is because the killed person was eager to kill his companion.