Is the prayer of an imam valid if he forgot a verse from Al-Fatihah after turning off the microphone, then made many movements to remove the hanging microphone, and did not perform the prostration of forgetfulness (sujud as-sahw)?

If the Imam forgets a verse from Al-Fatihah, then remembers it before bowing, his prayer is valid. As for the Imam bending to turn off the loudspeaker, his prayer is valid, as long as it does not happen twice. If it happens twice, then it becomes invalid. It might be severely disliked (makrooh shaddadatan) if the bending was significant. The movement that invalidates the prayer is one that is considered long by custom, without necessity, and continuous.