Is it permissible to shorten and combine the Dhuhr and Asr prayers after the Maghrib prayer time has begun, upon returning from a journey, and is it obligatory to re-perform them in full?

The Dhuhr and Asr prayers must be performed within their prescribed times. It is permissible to combine them during travel, either by performing them earlier (jama' taqdim) or later (jama' ta'khir). Delaying them until Maghrib is a grave error and a sin. As for making up missed prayers from a journey while one is a resident, there is a difference of opinion. The Shafi'is and Hanbalis hold that they should be made up in full. However, the Hanafis and Malikis believe it is permissible to shorten them. The preferred and more cautious approach is to make them up in full.