ANSWER:

You shouldn’t join Isha to Maghreb unless you are outside your own city boarders. You can pray it alone or on the plane. 

There is this musalla in the airport, but it is outside of the customs and immigrations. time of Isha is 10 mins before boarding time. I might be late for the boarding time, due to queue and custom/immigration if I perform Isha in the musalla, possibly with the jama’ah. Can I perform Isha Alone, before the boarding gate or pray Isha after magrib?