Is the Maghrib prayer valid if the worshiper intended it as the missed Asr prayer, and then prayed Maghrib alone?

Your prayer is valid. Missed prayers must be made up immediately and in the order they were ordained by Allah, for the Prophet, peace and blessings be upon him, said: "Pray as you have seen me praying." A difference in intention between the Imam and the congregant does not affect validity. However, if the time for the current prayer is short, it must be performed before the missed prayer.