Is it permissible for me to join the Imam in the Isha prayer, which he combines as a jam taqdeem with Maghrib, without a valid excuse, intending it to be the sunnah rawatib of Maghrib, and then leave him in the second rakah, knowing that the Imam is combining due to cold weather that is not severe?

Cold alone is not one of the causes that permit combining prayers. So, if the Imam combined Maghrib with Isha because of it, he would have prayed Isha before its time, and your prayer behind him in such a situation is invalid. Therefore, the Maghrib Sunnah (definition) prayer is not sufficient for you. And making up supererogatory prayers is legislated according to some scholars.