It seems like your question is specific. "Why do you need a ballast resistor with a 12V battery and a 6V starter." I don't think anyone has said you do. That's the first thing to get thru to you. If you will read the responses. HRL said a bypass system is "available" and TC said "Possible".
A full 6V system does not need a ballast resistor (I finally got that message, I think?.) A full 12V system operates better and longer with a two path circuit, with one path as reduced voltage to the spark generating components. The other a temporary "start/full 12V" path. It does not need the resistor to "operate". IF coil does not over heat, IF other components last, IF motor starts and runs OK. Just no reason not to have it on a full 12V system.
As far as the hybrid system you ask about. A person enters into it at their own risk. The experts do not know what other components are 6V and which are 12V. They don't know if the hybrid system person used a coil that is external, internal or eternal resistive, they don't know if it's pertronix, HEI or standard, They may not care to speculate on mix and match systems. They are Hot Rodders from way back and mechanics who did a lot of tricks for the drags and such. You can't learn from them if you attack them. They don't need you OR me. You are still a "grasshopper" with a web site.
The basic idea for the bypass is described in this thread for all others to decide how they want to design their hybrid system.(conversions).
The baseline for 6V and 12V is to do it stock.
