Can Muawiyah ibn Abi Sufyan's actions, such as his firm adherence to his position that led to Muslim infighting, and his making the rule hereditary for his son, be considered ijtihad?

No one among the scholars has claimed the infallibility of the Companions, including their foremost figures and pioneers. The reason for the fighting between Ali and Muawiyah, may Allah be pleased with them both, was the killing of Uthman and the demand for retribution for his blood. As for Muawiyah appointing his son Yazid to the caliphate, it was undoubtedly a mistake, as there were among the Muslims those who were more deserving and more righteous. However, this mistake does not negate Muawiyah's virtue, may Allah be pleased with him.