effte gets my vote for the most accurate response. My only comment would be that each magneto provides spark for all cylinders, so the spark from the booster coil would also be available to all cylinders. To help simplify.
1. Booster coils receive their voltage from the battery, usually though the starter switch.
2. Booster coils are used to provide high energy ignition spark for starting. (magnetos don't work well at low RPM's).
3. Booster coils provide a retarded ignition spark, usually close to Top Dead Center. (Aircraft engines do not have variable ignition timing like automobiles. The timing is fixed for optimum performance). Engines that have a high cranking RPM can start without a booster coil, but usually have one magneto retarded a few degrees more than the other. The Merlin with it's slow cranking speed would never start without a booster.
4. Would the Merlin run smoothly on the booster? I do not have direct experience here, but I seriously doubt it. On the radial engines that I work with that have a booster coil, you can get it to pop, but you had better have the mags on if you wanted to run.