Is it permissible to combine the Maghrib and Isha prayers at home due to rain, and is it necessary to repeat the Isha prayer if combining them is not permissible?

The jurists differed on the ruling of combining prayers due to rain outside the mosque. The more cautious approach is not to combine prayers in the future except in the mosque. You are not required to repeat the 'Isha prayer that you combined with Maghrib as an early combination, because you did what some scholars permitted. When an ignorant person performs an act without seeking a fatwa, and their act conforms to a saying of the people of knowledge, then that suffices them.