Just a few comments...."third heaven" is no mystery....it belongs to the schema of four or seven heavens that Jews and Christians at the time believed existed. Paul's readers would have certainly understood what the phrase referred to. There are many, many statements by Jews and Christians of the first and second century AD concerning the "Paradise of delight" (= Garden of Eden) being located in heaven and similar statements are found in the rabbinical writings. There are two main understandings: (1) That the original earthly garden was not destroyed in the Flood but was preserved in heaven, or (2) The earthly garden was merely a copy or shadowing of the more glorious paradise in heaven (in a rather similar way that the Temple on Mount Zion was a representation of the heavenly Temple).
Second, the NWT rendering of Ecclesiastes 9:5 of the dead being "conscious of nothing at all" is something of an overtranslation; unconsciousness is not quite the same thing as not knowing anything. Also the verse is usually used selectively as a prooftext; the same passage also states that "they have no further reward" and "never again will they have a part in anything that happens under the sun". This represents quite well the views of the Sadducees about death; the book is probably a proto-Sadducee work (one of the only works to survive from this branch of early Judaism). Such statements contrast with Pharisee and Essene affirmations of life after death, particularly, the belief in a future resurrection (as in 1 Enoch, Daniel, 2 Maccabees, 4 Maccabees, etc.).