2a. Through the knowledge that readers are supposed to be familiar with events “before time” we can infer that Genesis does not provide an account for the existence of all things, but rather only those things which it details. That is to say, there may be supernatural creatures that existed prior to God’s creation of material reality. So when we see references to certain creatures it is not to be assumed that they were created within the 6 days in Genesis 1.
Additionally, it seems as though some polytheistic influence can be detected here. After all, if beings existed prior to God’s creation of the material world and humans and those things came into conflict with God, it seems to stand to reason that people may have assumed that God did not create these beings but that they had a different origin. This would be akin to the Enuma Elish story where Marduk created a large part of material reality but other beings existed independently of him. This is not to suggest that Genesis and the Bible justify a worldview that says God did not create all things but merely indicates the possibility of influences on the thoughts of those who created the Pentateuch by polytheistic traditions.
2b. Source criticism may tell us that unexplained beings within the Bible were explained by the original source documents but that those explanatory portions have been omitted from the synthesis. For example, one source may have gone into detail giving an account of the origin of Leviathan or the cherubim but those details might have been omitted if they were deemed unimportant or at odds with certain theological views. However, later portions that make reference to these beings may have been retained because they were seen as significant leaving an apparent hole in the story. This explanation resembles Mobley’s because it says that explaining the origin of these beings was considered unimportant and thus omitted but it is distinct in that it presumes the explanation was once present but was edited out instead of omitted by an original author.
These two approaches do not seem necessarily incompatible. It may be that one source document explains Leviathan’s origins but omits details about the conflict between God and Leviathan since everyone may have already known. The initial omission and later editing out can both coexist.
Both approaches may also be a source of frustration for a modern reader. A reader might wish she knew the background story that is being assumed and may also wish that she had the primary sources for an account of an event. While on one hand these approaches help to explain certain things they also raise new questions for readers that we may not be able to find an answer to.