Is the prayer valid if the imam says the Taslim, then one of the congregants comes forward, doubting his ablution, and completes the prayer with the congregants after the imam has finished Surah Al-Fatihah?

If it becomes clear to the Imam that he has invalidated his ablution, his prayer becomes void, and he does not need to say the taslim. Appointing a successor is permissible, and the successor continues the prayer according to the Imam's established order, and is not required to re-recite Al-Fatiha.