Is it permissible to shorten and combine the 'Isha and Maghrib prayers before traveling a distance of over 300 km, knowing that most of the road is desert and the car does not stop?

It is not permissible to shorten the prayer (qasr) until after one has commenced the journey and passed beyond the houses of the city, and this is the view of the majority of scholars. As for combining prayers (jam‘), it is permissible before starting the journey if there is a need, and if separating them would cause hardship. This is the choice of Ibn Taymiyyah. Upon commencing the journey and passing beyond the houses of the locality, both shortening and combining prayers become permissible. If it is impossible to stop for prayer during travel, one must pray inside the car in whatever manner is possible, out of respect for the prayer's prescribed time. If the place of departure is not one's usual place of residence, it is permissible to shorten and combine prayers before departing from it.