Is it permissible for an employee who works 12-hour shifts daily for seven days, and lives in another city to which he only returns at the end of the week, to combine the Dhuhr and Asr prayers, given that he faces difficulty performing the prayers in congregation at their prescribed times?

It is obligatory upon those who work to perform prayers at their appointed times, and their work should not distract them from obedience to Allah. If one misses congregational prayer despite striving and taking all necessary means, then there is no blame on that person. Whoever travels and intends to stay for more than four days, then they are considered as a resident and must complete the prayer; it is not permissible for them to combine prayers due to travel. If it is highly probable that waking up for Dhuhr prayer will be difficult, then there is no harm in delaying it and combining it with Asr prayer as a delayed combination, provided that the difficulty is genuinely present and this is not a continuous practice.