Re: wild rotation?
I'm kind of with others in wondering why the big solid backplate too. The rocket is probably going to provide a hell of a thump to that plate when it goes off, and I can't see how it'll do anything but push the whole platform back, especially if there's a halfway-reasonable rocket engine providing the punch.
Now you could possibly re-use exhaust gas pressure, but that would involve swing wing designs or some way of slotting the whole thing inside a launch tube. Engine goes off, provides pressure in the tube which should fire the rocketplane out of the end like a bullet from a gun. Wings can flip out and sabots can fall off as the aircraft leaves the barrel. Methinks the SPB team are going for something with less moving parts though, as lovely as the idea sounds, and as epic as the launch footage would be.