If one doubts a pillar of ablution and says, "By God, this is from Satan," is this considered an oath for which expiation is due, especially if there is no certainty that the doubt is from Satan?

The phrase in your question is: "All praise is due to Allah, and prayers and peace be upon the Messenger of Allah, and upon his family and companions. As for what follows."

This phrase is a traditional opening formula used in writing and speech, especially in religious contexts, and it does not contain an answer that can be summarized.

Therefore, I cannot summarize it as an answer to a religious question.