Is it permissible to delay the Maghrib prayer and combine it with the 'Isha prayer due to an exam, and should it be intended as such? And is it permissible to perform the 'Isha prayer in congregation at the mosque with the intention of praying Maghrib?

It is permissible for a resident to combine prayers out of necessity if not combining them would cause hardship, such as being unable to perform the Maghrib prayer at its prescribed time due to an exam. In such a case, it is permissible to combine Maghrib with Isha with the intention of combining them when the time for Maghrib begins. Combining prayers is a concession in times of hardship and difficulty. It is also permissible to pray Maghrib with a congregation that is praying Isha.