Is it considered a sin and wrongdoing to rule for oneself the permissibility of swearing by other than Allah, such as saying: "By your life," despite prior knowledge of the prohibition of swearing by other than Allah?

Swearing by other than Allah is a minor form of polytheism (shirk asghar), a statement that is not permissible in Islamic law, and for which a servant will be held accountable if he utters it. As for thoughts and notions that cross one's mind, a Muslim is not held accountable for them, as Allah has overlooked such matters unless acted upon or spoken.