Is the prayer of one who performed ablution and led the prayer as an imam after a light sleep in the mosque, not knowing whether he had nullified his ablution or not, valid, as well as the prayer of the congregants, especially since if he had remembered a fatwa by a colleague sheikh – stating that cleansing after passing wind is only required if a sound occurs – he would have performed ablution as a precaution?

What your colleague heard from his shaykh is incorrect. Istinja’ (cleaning oneself after relieving oneself) from passing wind is not prescribed in the first place, let alone being obligatory. And istinja’ after sleep is not obligatory by consensus. So, what you did by limiting yourself to ablution (wudu’) is correct, and istinja’ was not required of you after waking up. Your prayer and the prayer of those who prayed behind you are valid, Allah willing, as long as you performed ablution after sleeping. Scholars have unanimously agreed that istinja’ is not obligatory after passing wind or sleeping. Imam An-Nawawi said: “Scholars have unanimously agreed that istinja’ is not obligatory after passing wind, sleeping, touching women, or touching the private parts.” We do not know of any disagreement on this matter, because istinja’ was legislated for the removal of impurity, and there is no impurity here.