Your post contains a lot of good questions, but I won't discuss all of them, only the first one.
The story of David and Goliath has been subject to a lot of analysis, since there are two different versions of the same story in the LXX and the MT. The latter is actually twice as long as the former. This and many other contradictions in the text led many scholars to conclude that the MT is combination of two different versions (this I think is the strongest proof that the OT has been composed of many different texts and authors).
There are a lot of discrepancies between these texts and I cannot list them all, but one important element lacking in the MT from verses 12-31 is David's status as Saul's harpist. In this version (but not the only version in the MT) David is still unknown to Saul and does not serve in the Israelite army, his presence in the battlefield is explained as him being a delivery boy. Verses 55-58 belong to this version, that is why Saul asks Abner after David slays Goliath "who is this kid?" However, in the LXX there is only one version. There David is known to Saul and is actually his armor-bearer, David volunteers to fight Goliath and eventually slays the giant. The story runs more smoothly and verses 55-58 are missing.
Hope this answers your question.
For a fuller discussion of the differences between the versions of the Goliath narrative in the MT and LXX see this.