We are going to differ on our interpretations on Ecclesiastes based on what you have written here. I am taking the hermeneutical approach that Ecclesiastes is a member of the books of wisdom (Psalms, Proverbs, Ecclesiastes, and Song of Solomon). With this in mind I also affirm that these scriptures were God-breathed and useful. Every single writer of Scripture was a completely sinful person in need of God’s grace and brought their flaws to their writing. I think it is incorrect to throw out Ecclesiastes’ teachings based on the sinful lifestyle of Solomon. Otherwise we need to get rid of the Septuagint’s teachings — written by a murderous leader. Or perhaps remove ourselves from the wisdom of the Psalms since they were written by an adulterer. How can we trust the theological treatise of Romans since it is written by a bonafide Christian killer?
We are given the book of Ecclesiastes as a book of wisdom, and it is my understanding that it would be wise of us to heed the teachings there in, and submit ourselves to the mysteries they uncover, rather than trying to pick and choose what passages best support our understanding of God.
Reading the Bible has only made me less certain of who God is. And this leads me to wonder, and it leads me to worship. I hope that helps you understand where I am coming from, and why we might be led to disagree on interpretation.