Is combining the Isha prayer due to rain permissible or invalid, and do the worshippers bear sin if it is not permissible, and what is the ruling on someone who prayed Maghrib and Isha combined then prayed them again in their due time in congregation at another mosque?

Combining the Maghrib and Isha prayers due to rain that wets the clothes is permissible. A condition for this is the presence of rain at the beginning of both the first and second prayers. If the rain stops before the start of the second prayer, the combining of prayers is not valid, and the Isha prayer, having been performed before its time, is considered invalid and must be repeated. The Malikis, however, are of the view that the cessation of rain after commencing the Maghrib prayer does not affect the validity of the combination.