Does the prayer of a congregant become void if he intended to stand with the imam who forgot the first Tashahhud and remained standing, then the congregant changed his mind about standing and sat down when the congregants reminded the imam, and the imam returned and sat with them?

If the Imam forgets the first tashahhud and stands up completely, he should not return [to the sitting position]. Instead, he should complete his prayer and perform the prostration of forgetfulness (sujood al-sahw). If he does return after having stood up completely, and he did so out of ignorance or forgetfulness, his prayer is valid. However, if he did so intentionally, his prayer is nullified. The congregants (ma'moomeen) must follow the Imam, and they are excused if they are unaware of the ruling. The nullification of the Imam's prayer in this situation does not extend to the prayer of the congregants because they are not aware of the reason [for its nullification].