Are the prayers and fasting of those who combine the Maghrib and Isha prayers, delay the Tarawih prayer until the time of Isha, and follow the Turkish calendar for Fajr, valid?

Isha prayer must be performed within its prescribed time. It is permissible to combine Maghrib and 'Isha prayers by advancing the 'Isha prayer for those who find it difficult to wait for its time, based on the saying of Ibn 'Abbas (may Allah be pleased with him): "The Messenger of Allah (peace and blessings be upon him) combined the Dhuhr and 'Asr prayers, and the Maghrib and 'Isha prayers, in Madinah, without fear or rain." When Ibn 'Abbas was asked why he did that, he replied: "So as not to cause hardship to his Ummah." However, it is not permissible to combine the prayers for those who do not experience such hardship.

If you combine Maghrib and 'Isha prayers in Ramadan by advancing the 'Isha prayer, Tarawih prayer should be performed immediately after the combination, because combining the prayers makes the two prayer times into one.

There is no harm in following the Turkish calendar for Fajr prayer. However, it is more cautious to stop eating and drinking for fasting when the sun is at 15 degrees below the horizon. As for the prayer, there is no harm in delaying it until the sun is at 13 degrees below the horizon, or even later than that.