What is the legal ruling regarding the Eid prayer in which the Imam said the takbir for standing in the second rak'ah, then immediately started reciting Al-Fatiha, then realized his mistake and returned to the five takbirat, and then recited Al-Fatiha thereafter? Is the prayer considered valid, and what is the evidence from the Sunnah for that? Does the Imam prostrate for forgetfulness (sujud al-sahw)?

The more correct view is that if someone forgets the Eid takbeers and remembers them after starting Al-Fatiha, they should not repeat them because their place has passed, as they are a Sunnah (definition) whose place has elapsed. If they remember them during or after rukoo', they should not repeat them, and there is no disagreement on this. If they remember them after the recitation and before rukoo', the more correct view is that they should not say the takbeers because their place has passed. If they remember them while reciting Al-Fatiha, they should not repeat them according to the new opinion because their place has passed, whereas, in the old opinion, they would repeat them and then restart Al-Fatiha. The prayer is valid in all cases.