What is the ruling on the imam changing his sitting posture after the prayer and turning to face the congregants, and must he sit on his knees, like the "iftirash" posture, out of respect for the pulpit of the Prophet (peace be upon him)?

The Sunnah is for the Imam to turn his face towards the congregants after concluding the prayer with taslim, and after saying "Astaghfirullah" (I seek forgiveness from Allah) three times, and "Allahumma Antas-Salam wa minkas-salam, tabarakta ya Dhal-Jalali wal-Ikram" (O Allah, You are As-Salam, and from You is all peace, blessed are You, O Possessor of Majesty and Honor). It is permissible for him to turn away to his right or his left.

As for the manner of sitting, the matter is broad, as no text has specified a particular posture for sitting. Imams sit cross-legged and not cross-legged, and there should be no artificiality in this matter or disapproval of a customary posture.