It actually is the whole story. There's no magic in case design that can get away from the fundamental physical science at work. An action creates an equal and opposite reaction. That's recoil. There's no spooky action at a distance, no relativity to consider--it's just boring old Newton.
Not really, no it isn't. Wide powder columns may burn more _uniformly_, but they do not burn any faster or slower than narrow powder columns. Powder burn rate is a physical constant, based on the properties of the powder and the starting point of ignition (the primer).
No, it's not. 'Efficiency' means 'how much energy comes out (MV and ME) versus the amount of energy that went in (joules of chemical energy--the powder charge). Winchester marketing used this term to explain how the .300 WSM could produce identical velocities to the .300 Win Mag with less powder capacity. They used it correctly, and it's a fine (if slightly misleading) explanation for what's happening.
This is totally untrue. All powders but particularly progressive powders (i.e. 'slow' or 'magnum' powders) do some burning in the barrel, and that's exactly what you want--you want the pressure spike to rise smoothly and evenly until the moment the projectile leaves the barrel. Even a .22 LR needs at least 12" of barrel to burn all the powder in its miniscule case. Again, the burn rate of powder depends entirely on the physical characteristics of the powder and the initial ignition, not on the case design.
It may burn less uniformly, but it will not burn any faster or slower, and it will not transform the chemical energy of the powder into bullet velocity any more efficiently. These things are simply not possible.
It doesn't matter where the powder burns, case or throat or barrel, it all creates energy which pushes the bullet out, and pushes back against your shoulder.
This is true! Barrels wear out from hot gasses eroding away the rifling at the throat. Note that all cartridges expel hot gasses into the throat, even magical short fat ones.
Yeah, wonder why that is....?