Is it permissible to perform Tarawih prayers behind an Imam who prays Isha before its due time, on the pretext that the interval between the Maghrib and Isha adhans is only an hour and a half, making the sky appear as if it is still Maghrib time?

It is not permissible to perform the Tarawih prayer before the 'Isha prayer and its time has begun. However, given the late entry time for 'Isha, it is permissible to combine the Maghrib and 'Isha prayers by performing them earlier (jama' taqdim), and then perform the Tarawih prayer thereafter.