If a person enters the mosque and finds the congregants praying 'Isha' with a Jam' Taqdeem (combining prayers by bringing the later prayer forward) due to rain, should he pray 'Isha' with them, then pray Maghrib individually, and would his prayer be valid, or should he repeat both prayers in their proper order after completing the 'Isha' prayer with the congregation?

For the Jam‘ al-Taqdim (combining two prayers in the time of the first), tartib (observing the correct order) is a condition. If you enter the mosque and they are praying ‘Isha’ while you intend to perform Jam‘ al-Taqdim, and you pray ‘Isha’ with them, then you must pray Maghrib first, and then ‘Isha’ again. This is because the first ‘Isha’ prayer does not fulfill the obligation due to the absence of the condition of tartib. Some Hanbalis, however, hold the view that the condition of tartib is waived in cases of forgetfulness.