Is it permissible for a resident who has no excuse to combine Maghrib and Isha prayers with the mosque congregation that combines them on the pretext of the late time for Isha, or should he pray Maghrib with them and then return to pray Isha alone or with a small group? And is it permissible for him to pray the regular two rak'ahs (sunnah) alone after Maghrib while they are praying Isha in congregation?

If the time for the 'Isha prayer is known, the default is to pray it at its designated time. It is not permissible for a resident to combine it with Maghrib except when there is a need, and provided that this does not become a habit. If there is no excuse to combine 'Isha with Maghrib, then do not combine them. It is preferable to leave the mosque and pray supererogatory (nafl) prayers at home, or to pray 'Isha later in congregation with family or individually after its time has begun.