What is the ruling of Islamic law in a case where one cannot recall the details of a specific incident or the exact word that was spoken, and is it obligatory to ask those who were present during the incident, knowing that the questioner suffers from obsessive thoughts regarding his divorce of his wife based on a word he uttered before consummating the marriage?

What you have mentioned has no consequence and does not result in a divorce. The fatwa (definition) given to you by the aforementioned Shaykh is correct. Therefore, do not pay attention to these whispers (waswas), do not think about this matter, and do not ask about it again. Turn away from the whispers no matter how they come to you or in what form they appear, whether they relate to matters of creed, worship, divorce, or anything else. Indeed, these whispers are from Satan, who casts them into your heart to disrupt your life. So, do not give them any importance or pay any heed to them.