What is the ruling on the prayer of one who intended to pray Maghrib silently while leading the Dhuhr prayer, and what are the implications for the congregants?

Uttering the intention when beginning prayer is not legislated. What matters is the intention of the heart. So, if the worshipper vocalizes an intention different from what he intended in his heart, what counts is what he intended in his heart, and this does not affect the validity of the prayer.