Is it obligatory for me to pray 'Isha in congregation, or to pray Maghrib alone and then 'Isha upon returning from travel and catching the 'Isha prayer in congregation before performing the Maghrib prayer?

If you enter the mosque and the Imam is praying Isha, and you have not yet prayed Maghrib, then join the Imam with the intention of praying Maghrib. When the Imam stands for the fourth rak'ah, you should sit for the third rak'ah and complete the tashahhud and du'a. Then, wait for the Imam to say the taslim and say it with him. The difference in intention between the Imam and the follower does not matter.

It is permissible for you to wait in the tashahhud until the Imam finishes his prayer and says the taslim with him, then pray Isha. Alternatively, you can pray Maghrib alone and then join the congregation for the Isha prayer.