To your first question, I think the time period the reader lives impacts how they see the genre of the text in this case. A particular genre also doesn't necessarily make everything contained with in it purely symbolic, symbolism is just one literary technique among many that can be used to get your message across. For modern readers even the translation you're using matters, as in is it a translation from the masoretic text or septuagint? It affects how the first line is interpreted and I can see how "In the beginning" in English can make one expect a straightforward account of the creation as it happened. Also are we including knowledge of Ugaritic or Babylonian or Sumerian literature as derived from mainstream science because these can affect how we view its genre as well. It is clear that the intended audience the author(s) are writing to did not have access to what we now know about science and that definitely needs to be taken seriously. How is the word modern to be interpreted? Those passages that seem to indicate that the structure of the world we live in consists of a solid dome shaped barrier that holds back waters above us and the earth below holding back waters below us, where the world is essentially a flat circle aren't given a straightforward literal reading and people don't seem to have any problem with this (unless it's Spherical). Do these accounts make sense on their own? That's kind of a personal call, does it make sense to have two creation accounts that are meant to be taken totally literally which seem to conflict on some points? Here again a lot of YEC are willing to deviate from a straightforward reading to make sense of this, I think that's necessary on some level.
What would the people at the time of this writing make of it? That's what really matters, I think, if we want to arrive at the intended meaning of the Genesis creation account. We have reason to believe that certain creation myths, and I'm using this word in a narrowly defined way, from this general region of the world were widespread at this time. Here by myth I mean basically stories being used as vehicles for communicating some truth in a way analogous to the parables of Jesus. There are real similarities that suggest that it was common to take an earlier myth and incorporate it into a new myth while changing it substantially. A good example would be the story of Atrahasis being recycled in the story of Gilgamesh, where Atrahasis can be linked to the character Utnapishtim, and a parallel can be drawn to the story of Noah as well. I think the biblical audience at this time period as well as the authors were well acquainted with these stories. Not only that they represented competing religious ideologies for the Jews who were keen on distinguishing themselves from the Canaanite religion still present in the area. For this reason, it is not the similarities that really matter, I'm not accusing these authors of plagiarism, but instead the huge differences you see along with the similarities. I think this was a strategy to use motifs and themes common in Babylonian and related Semitic religions to tell their own creation myth in a way that clearly distinguishes it from other ideas about human and divine origins. This may also have been a strategy to win converts from these religions. In the case of the creation account in Genesis these parallels occur with Enuma Elish and related stories, which I think don't just happen to present an identical model of the structure of earth. I think this is evidence that even ignoring scientific problems this story creates for a literalist, a typical Jewish listener would recognize this immediately and understand that this is a creation myth similar to others they've likely heard before so the genre is fairly clear here.
Do I see the bible as supernatural in origin? I tend to see it as a very human collection of writings that present the Jewish and later Chrisitian views of the nature of God, though when God is literally quoted I think there's a case to be made that it's supernatural in origin. Do I believe that "sin entered into the world through one man"? I think the Genesis account of the fall of man is fundamentally about the origin of sin in the world so on that level yes. I just don't consider Adam to be a historical person, or even a proper name since it just means man.