Is what the questioner did—shortening and combining prayers—considered a mistake, and is he required to make up the Maghrib and Isha prayers, in addition to past prayers before his residency in Al-Muthnib?

The distance for qasr (shortening prayers) is set at approximately 83 kilometers. If your village is 90 kilometers away from Unaizah, then you are permitted to shorten and combine prayers when traveling to Unaizah, as well as when passing through Al-Midhnab or any other village on your way. There is no sin in not shortening prayers, although doing so is better. If you settle in Al-Midhnab, it becomes your home (watan), and therefore you are not permitted to combine and shorten prayers when traveling to your previous village if the distance is only 60 kilometers. If Unaizah is less than 83 kilometers from Al-Midhnab, then you are not permitted to combine and shorten prayers, and in this case, the shortened Isha prayer must be repeated.