Is it considered legitimate in Islam for the congregants to remain seated after the prayer until the Imam stands up, and should the Imam therefore not prolong the post-prayer remembrances (dhikr) that are recited after the Fajr and Maghrib prayers, out of consideration for this?

It is not disliked for the congregant to stand before the Imam if there is an excuse. However, if the congregant departs before the Imam completes his remembrances and before the Imam stands, without an excuse, then he has gone against what is better. The Imam is required to change his position after the salutation, not to stand before completing his remembrances.