I don't have the answers to these questions, but I have a really cool clue.
The Noam Elimelech (פרשת וארא) talks about our Fathers, Avraham, Yitzhak and Yaakov, and I'd like to try to develop and flesh out his words here. He explains that until our three Fathers came on the scene, when someone wanted to connect to God, they needed to ascend into the heavens, and relate to God on that level. As a result, very few were able to attain such revellations.
Our Fathers each bound themselves to one of HaShem's Attributes until all of their actions, all of their being embodied that Holy Attribute. Avraham revealed HaShem's Hesed, His Kindness, Yitzhak His Gevurah, His Might, Yaakov His Tiffereth, His Perfection. Each of these attributes, brought down from on high, revealed in this world by our Fathers is now a way that we can connect to HaShem without having first to traverse the heavens.
In essence, we can grasp onto these attributes down here in this world, and hold on while they return to their heavenly origins, in effect hitching a ride to a heavenly audience with the Master and King of The Universe.
Each attribute is a sort of spiritual sling. The tension of their existence down here means by just connecting to our Fathers, we can be catapulted heavenward. This is the meaning of the first berachah in the silent Amidah, אלוקי אברהם אלוקי יצחק ואלוקי יעקב הקל הגדול הגיבור והנורא - the God of Avraham, God of Yitzhak, God of Yaakov, The Great, Mighty, and Awesome God. First we mention our three Fathers, then we mention the three corresponding Attributes of God. [Great = Kind, Mighty = Mighty, Awesome = Perfect to match up my earlier definitions with my later translations] We arrive at a His attributes through our fathers.
What does this have to do with David HaMelech? It seems to me that this was the secret weapon David brought to the battle. Goliath was a towering giant. David was a young man. By choosing the sling as his weapon David said this: You might seem mighty, but I, no more than a simple stone, am empowered through the sling of my Fathers. I can reach infinitely higher than you, giant, simply by seeking God through the connection I inherited from my ancestors.
For this, David who understood that prayer--connecting to HaShem--was beyond any earthly power, merited to save his people, become King, and compose the most complete book of prayers (Tehillim) ever written.
David, Our Rabbis teach us, was one of the seven shepherds, each of whom embodied an attribute of HaShem.
- Avraham - Hesed
- Yitzhak - Gevurah
- Yaakov - Tiffereth
- Mosheh Rabeinu- Netzah - (eternity/victory)
- Aharon HaKohein - Hod - (majesty)
- Yosef HaTzaddik - Yesod - (the foundation pillar)
- David HaMelech - Malchuth (Kingship/Dominion)
Malchuth which is simultaneously the lowest and the highest of the attributes, is the ultimate slingshot through which we can seek out HaShem in all of his inner most chambers. Not surprisingly the Tzemach Tzedek said about Tehillim that if we knew the power the words of Tehillim have, we would be saying them all day long.