What is the ruling regarding two prayers: the first (after the Imam combines Maghrib and Isha in advance due to light rain), and the second (after re-praying Isha in another mosque)? And what is the ruling regarding the Maghrib prayer?

The rain that permits combining prayers is that which wets clothes and causes hardship, not light rain that does not wet clothes. If the Imam combined prayers due to rain of this description, then the 'Isha prayer is valid. Otherwise, the repeated prayer is the valid one. In both cases, one of the two prayers sufficed and the other is supererogatory. The Imam should be advised if he combined prayers when it was not permissible.