What must one do who entered the 'Isha prayer with the intention of Maghrib due to a legal excuse, then stood up to pray 'Isha without performing the taslim for Maghrib, and remembered this after performing the taslim for 'Isha? And what should he do if he remembers this during the 'Isha prayer?

If you stood up for the 'Isha prayer before completing the Taslim of the Maghrib prayer, then the 'Isha prayer is considered invalid because you entered into it while still engaged in the Maghrib prayer, before a long separation. Scholars differed in defining "long separation"; some linked it to custom, others to the duration of the prayer in which the error occurred, and some to the length of a single rak'ah. The Shafi'is considered two rak'ahs a long separation, and the Malikis considered anything more than reciting Al-Fatiha. Based on this, the separation in this case is long, and you must repeat both the Maghrib and 'Isha prayers if you remember after the Taslim of 'Isha. However, if you remember before the Taslim of 'Isha, and if that was after completing two rak'ahs, the ruling is the same. But if it was before that, then go back and perform the prostration of forgetfulness (Sujud as-Sahw), and then perform the Taslim of Maghrib, and then pray 'Isha. The more cautious approach is to repeat both Maghrib and 'Isha if you exceeded the recitation of Al-Fatiha in the second prayer.