DJW,
All this has been covered before. Ecclesiastes is a book that is part of the "WISOM LITERATURE" in the OT. It is written from a carnal point of view, same as an atheist.
From this carnal, purely physical point of view, life is all ultimately meaningless. It is a worldview that the writer ultimately rejects after considering all things. This is why scriptures like Eccl. 10: 19 which reads, "money is the answer for everything", are not projected into a Christian worldview. There is no Christian doctrine that teaches money is the answer for everything even though this is stated clearly in Eccl. 10: 19 .
"Therefore I hated life; because the work that is wrought under the sun is grievous unto me: for all is vanity and vexation of spirit." But, Jesus said we can have meaning in our life and peace. Again, different points of view.
The Bible tells us in 1 Thessalonians 5:23 that we were all created with three parts—a spirit, a soul, and a body: Only one exists "under the sun".... the body. When the body becomes separated from the soul, a death occurs. Biblically, "death" MEANS SEPARATION, not annihilation or extinction of the soul/spirit. (Gen. 35:18)
Taking a rejected (carnal or purely physical) worldview and making a Christian doctrine out of it requires not only rejecting the book's own stated framework in which the book is to be understood ("under the sun"), it also requires ignoring or waving off dozens of other scriptures that show people as fully conscious after their consciousness (soul) separates from their body.
I posted a number of these scriptures earlier in this thread.
As far as the resurrection goes, most scholars (skeptic, Jewish, Christian etc.) who publish in this area agree on these minimum facts that are well attested to in history.
Many people find these facts impossible to explain away from a purely natural point of view. So, they end up believing that Jesus was raised from the dead. If a person believes that, then it provides a formidable basis to believe the other things that Jesus said.
First century religious leaders didn't know what to do with this man. So, they had him executed, but he apparently didn't stay dead. People today still wonder what to do with him. Wise men still seek him. Christians simply believe him and take him at his word.