The point is, there are a lot of design considerations coming into play here; it's not just one or two things.
You are dealing with big currents (Amps) and large voltages, which can both capacitively and inductively transmit to other parts of the circuit.
You have to consider avoiding loops in wiring which can pick up interference by induction, you have to consider shielding from capacitive pickup, you have to consider resistances in wiring across which voltage drops will occur and you have to consider insulation strength of the higher voltage parts.
The most important considerations are keeping the outline of the circuit physically small to reduce the resistance and inductive effects, and design the PCB to "quarantine" the high voltage end from the lower voltage end. It is simply not very practical to "breadboard" it with flying leads. You need a common or "star" ground at the PCB itself; whilst you generally cannot arrange the wiring to the coil, the Hall sensor wires must return directly to the PCB in a single close bundle with no other connection, as must any wiring to controls or display. Obviously you must provide generous wideband (electrolytic plus ceramic) decoupling on the PCB and on the display board.
You should not be using interrupt routines.
The resistor on the spark plug is indeed for RFI suppression. This is however not likely to relate to your problem here - it is not a radio and it is the lower induction frequencies that are a problem. In fact, to the extent that it (slightly) raises the voltage on the ignition lines and slows the impulse, it makes the problem worse.