Is the prayer valid for someone who intended to pray Maghrib, then intended to pray Isha, and completed it with the Imam, and what are the consequences of that?

If you intended to pray the Maghrib prayer, then changed your intention during the prayer to the 'Isha' prayer, both prayers are invalidated. The Maghrib prayer is invalidated because its intention was قطع (severed/broken), and the 'Isha' prayer is invalidated because its intention was not made from the beginning. You must repeat the 'Isha' prayer, and the Maghrib prayer if you have not already prayed it.