Pole,
You do raise essential questions (which perhaps would be worth another thread).
One problem is that our catalog of literary genres doesn't suit the ancient worlds and works. We have gradually (and still partially) separated "history" from "fiction" or "art" from "religion", as potentially "pure" types or categories, whereas in ancient cultures everything was related and largely undistinct. The gods of mythology were represented in statues and worshipped in temples with music and dance; the Greek theater emerged in a cultual and ritual setting. Even the concepts of "truth" or "believing" were different: this was very well demonstrated decades ago by Paul Veyne in his famous essay Les Grecs ont-ils cru à leurs mythes? (Did the Greek believe their myths?). Greek philosophy is perhaps the first methodical attempt at extracting a logos from the overwhelming mythos -- and this is the very beginning of our critical civilisation. But this critical analysis, for what it is worth, doesn't work backwards.
Now back to the highly elusive question of the author's intention. Frankly I doubt that the authors of the stories about Jesus being born from a virgin, walking on the sea, quieting the storm, transfigured or ascended to heaven would intend their texts to be taken literally as we often do; perhaps they couldn't even imagine that some people would read them this way. Those were texts for liturgical use (they were read aloud) and they were written to serve and develop a collective experience. If we read them as a newspaper report or an encyclopedia article (whether to defend them or to criticise them at this level of reading) we are to blame -- not the texts or the authors.
But there is much more to any text than its author's intentions. As Derrida pointed out in many ways, what the author writes is not just what s/he means. On the one hand s/he can be unaware of many literary influences which nonetheless are in his/her work; on the other hand no reader will ever understand the text exactly as s/he meant it. Writing is committing oneself to (and losing oneself in) the infinite drift of difference in scripture, which includes the endless potentialities of intertextuality.