Is it permissible for a traveler to combine the 'Isha prayer with the Maghrib prayer by performing them together earlier (jam' taqdim) and shortening it, or must it be completed on the road if the car stops at a rest area for 'Isha?

If you pray Maghrib before leaving the village, then it is not prescribed for you to advance the 'Isha prayer. Rather, it is obligatory to delay it and pray it on the road after its time has begun. It is not permissible to avail oneself of the concessions of travel (such as combining and shortening prayers) except after actually departing from the buildings and orchards of the town.