What is the ruling on the prayer of one who prayed Maghrib alone upon entering the mosque after the 'Isha'adhan and before its iqamah, then completed it and joined the congregation for the 'Isha' prayer, and is it valid to make it up with the intention of Maghrib?

What you did is correct, and there is no blame upon you for it. Your Maghrib prayer suffices for Tahiyyat al-Masjid (greeting the mosque prayer), and your prayer is valid and you are not required to repeat it. However, if you delayed Maghrib until its time expired without a valid excuse, then you must repent to Allah.