How can midnight be calculated based on the times of Maghrib and Fajr prayers, and is a ‘Isha prayer performed a short time before midnight (such as a quarter of an hour) or one that finishes exactly at midnight considered to be performed on time, and are there any consequences thereof?

Night begins at sunset and ends at dawn. Its middle is known by determining the time of sunset and dawn, then calculating half the sum of the hours between them, which in the mentioned case corresponds approximately to the end of the eleventh hour. The time for 'Isha' prayer extends until midnight, which is the preferred time for performing the prayer, as Al-Nawawi mentioned that the more correct opinion is that the preferred time extends until midnight. After that, it becomes a permissible time until the second dawn.