What is the ruling on praying Maghrib in congregation with a group praying Isha by way of الجمع تقديم (combining two prayers by offering the second at the time of the first), and is it permissible to join them from the second رکعة (unit of prayer)?

It is permissible to pray Maghrib behind someone praying 'Isha according to the Shafi'i school and those who agree with them, and this is the fatwa given. So, join them with the intention of praying Maghrib. If the third rak'ah is your last, then sit and wait for the Imam until he says the taslim, and then say the taslim with him. If you separate from him and say the taslim, there is no harm. There is no harm in joining the Imam in any rak'ah.