Is establishing the 'Isha prayer one hour or one hour and twenty minutes after the Maghrib Adhan, and before its prescribed time according to the Adhan schedules, considered part of the Sunnah or an act of ijtihad? And what is the ruling on praying at these times?

The times for prayers are only known through cosmic signs. The time for the 'Isha prayer begins with the disappearance of the red twilight. The period between Maghrib and 'Isha varies. Delaying the 'Isha prayer until the first third of the night or its half is preferable, provided there is no hardship and a congregation is present. Whoever is certain or predominantly believes that the time for 'Isha has entered, their prayer is valid. However, whoever is certain that its time has not entered, or doubts, their prayer is invalid.