Is a traveler's prayer valid if he completes Maghrib and shortens Isha upon his arrival, despite being a resident of the area? And is the prayer valid for someone who forgot Maghrib and remembered it at the time of Isha, so he prayed three rak'ahs with the Imam, then sat and said the taslim with him?

Travel concessions expire upon the traveler's arrival in their hometown. Therefore, whoever prays 'Isha with a shortened prayer after arriving in their residential area, their prayer is invalid and they must repeat it as four rak'ahs. As for praying Maghrib behind an imam who is praying 'Isha, it is valid according to the preponderant opinion that permits a difference in intention between the imam and the follower.