Is it permissible, after the taslīm of the Maghrib prayer, for me to intend the ‘Ishā’ prayer as a follower behind a person who started the Maghrib prayer with me and then stood up to complete the ‘Ishā’ prayer, in order to attain the reward of congregational prayer?

Scholars have differed regarding the ruling on the difference in intention between the Imam and the congregant (ma'mum). The most preponderant opinion is that it does not cause harm, and this is the madhhab (definition) of the Shafi'is and those who agreed with them. Accordingly, it is permissible for you to pray 'Isha behind someone who is completing the 'Isha prayer, and it does not matter that he initially prayed Maghrib behind you.