re: Propellant geometry
As I understood it, the blowout ports to shut off thrust do so by increasing the area where the gases can escape to the point where thrust from the nozzle area becomes neutralized. Kind of turning the thing from a firecracker to a "fizzler." Obviously, the propellant keeps burning to depletion.
The "Big Dumb Booster" idea, a large pressure-fed liquid with a simple injector, has performance comparable to that of a solid, and if designed properly, could be recovered in the same way and reused.
Also, LOX and kerosene, while not pollution-free, don't spit out the crap that a solid does, and refueling can be done easily on site.