Is it permissible for an employee whose workplace is 100 km away from his home, and who commutes there two working days and two days off, to combine and shorten prayers (such as Maghrib and Fajr), given the difficulty of performing prayers in a mosque along the way?

If the work distance is 100 km, then it is permissible to combine and shorten prayers (Dhuhr with Asr, and Maghrib with Isha) at the workplace or during the journey, even if the travel is repeated. There is no sin in a traveler missing congregational prayer, as he is excused.