(A) He's supposed to be on the verge of manhood. He's also been trained by the two greatest warriors in the known Imperium (Duncan Idaho, Gurney Halleck). The leader of the Sardaukar even mentions that the Atreides forces are the best fighting force there is (next to the Sardaukar, presumably). Paul has trained almost every day of his life in hand to hand combat. No telling how far the stories will go (there were 6 original books), but Paul needs to age about 10 years by the end of the first one, so the actor needed to look weedy in this one. That's one benefit of them not starting the next one until next year, the actors will all be 2 to 3 years older than they were in this one.
(B) The actress is 38, she's supposed to be the mother of a 15 year-old Paul at the beginning.
(C) Think of it more like oil beneath the sands of a bunch of nomadic desert tribes, and how the European powers carved up the territories for themselves. Herbert was also subverting that by having the Bene Dessert spend centuries planting the stories that would become "prophecies" for the Fremen. I think of it more as union of the two worlds (water + desert, European + Middle Eastern) that leads to the ultimate strength. Remember, Paul has visions of a holy war in his name burning across all the known worlds of the Imperium and killing billions of people. The story of Dune is as much of an indictment against imperialism as it is an indictment of capitalism and religion.