What is permissible for us in the mosque after combining Maghrib and Isha prayers, and do we leave immediately after the Imam's Tasleem without post-prayer remembrances, and is one who does that to be condemned?

It is recommended to recite the prescribed remembrances (adhkar) after prayer, and there is no distinction between the case of combining prayers and other situations. Thus, one should not condemn someone who recites them or abstains from them, as they are not obligatory. However, Maliki jurists disliked performing supererogatory prayers in the mosque after combining Maghrib and Isha prayers.