Who in your opinion (bearing in mind the internal evidences)wrote Ecclesiastes?
I think all agree that Ecclesiastes is written “as though” Solomon wrote it: Ecc 1:1 the Preacher, the son of David, king in Jerusalem. Ecc 1:16 Lo, I am come to great estate, and have gotten more wisdom than all they that have been before me in Jerusalem: yea, my heart had great experience of wisdom and knowledge.
Who else but Solomon could this be referring to?
But I still do not think Solomon actually wrote it. Primarily because there is no evidence of his repentance elsewhere in Scripture. 1 Kings 11:9-11 9-And the LORD was angry with Solomon, because his heart was turned from the LORD God of Israel, which had appeared unto him twice, 10-And had commanded him concerning this thing, that he should not go after other gods: but he kept not that which the LORD commanded. 11-Wherefore the LORD said unto Solomon, Forasmuch as this is done of thee, and thou hast not kept my covenant and my statutes, which I have commanded thee, I will surely rend the kingdom from thee, and will give it to thy servant.
Nevertheless, some would say that Ecclesiastes itself is all the evidence we need of repentance.
But as to your question, I have no idea who wrote the book of Ecclesiastes. Sorry I couldn’t be more help.