Is it permissible to feign witnessing someone sign blank papers and then swear to that in court, despite certain knowledge of his innocence, and what is the punishment for whoever does that if it is forbidden?

If the reality is as described in the question with absolute certainty, then there is no harm in using tawriya (ambiguous speech) in an oath to repel harm and injustice from a Muslim, as long as it cannot be achieved except through such an oath. In fact, it is recommended and is considered the essence of truthfulness, not a lie. The Prophet, peace and blessings be upon him, said to Suwayd ibn Hanzalah when he swore that he was the brother of Wa'il ibn Hujr to save him from his enemy: "You spoke the truth; a Muslim is the brother of a Muslim."