The Imam sitting down again after standing up and before starting recitation

Q 1: We were offering the ‘Asr (Afternoon) Prayer in congregation in the Masjid (mosque). At the end of the second Rak‘ah (unit of Prayer) and after performing Sujud (prostration), the Imam (the one who leads congregational Prayer) forgot to recite the first Tashahhud (a recitation in the sitting position in the second/last unit of Prayer). The Ma’mums (persons being led by an Imam in Prayer) did not follow him and remained sitting to recite the Tashahhud. Some of them tried to draw the Imam’s attention to his mistake by saying, "Subhan Allah (Glory be to Allah)." The Imam, thus, sat down to recite the Tashahhud and performed Sujud-ul-Sahw (Prostration of Forgetfulness) after saying the Taslim (salutation of peace ending the Prayer). However, a man got up and said that the Salah (Prayer) was Batil (null and void), because the Imam sat down again after standing up. He said that if just the hands and knees of the Imam had slightly risen off the ground, he should not have returned to sit down. He also said that the Ma’mums did not follow him when he stood up. As a result, some people repeated their Salah and others did not. Was the Salah Batil? Please answer in detail and may Allah reward you greatly! A: If the Imam has stood up fully erect to offer the third Rak‘ah having forgotten to recite the first Tashahhud and some of the Ma’mums have drawn his attention to his mistake, it is Makruh (disliked) for him to return to the sitting posture. If he has already started the recitation of Surah Al-Fatihah, then it is prohibited for him to return. When he concludes his Salah, he should perform Sujud-ul-Sahw before saying the Taslim. This is because the first Tashahhud is one of the obligatory actions in Salah that can be compensated for by performing Sujud-ul-Sahw. (Part No. 6; Page No. 25