What is the wisdom behind delaying the 'Isha prayer until just before Fajr, given that prayer is a timed ordinance, as stated in the noble Hadith: "Were it not that I would overburden my Ummah, I would have commanded them to delay the 'Isha prayer until just before Fajr"?

No hadith states that Isha prayer should be delayed until just before Fajr. Rather, it is recommended to delay it until the first third of the night or its half, as the Prophet, peace and blessings be upon him, said: "Were it not that I would overburden my Ummah, I would have commanded them to delay the Isha prayer until the first third of the night or its half." Delaying it is better for whoever it does not overburden, nor the congregants. This does not contradict the fact that prayer is "a book prescribed at fixed times"; for the meaning of "prescribed at fixed times" is that it is obligatory at a specific time, and Isha remains within its time if delayed until the first third of the night or its half.